Vacation time for parents can be any time of year their children are invited out of town -- alone. How you get your child to a destination can make the difference between a successful solo trip and a difficult situation for everyone.

One Washington mother panicked when her 7-year-old was invited by a family friend to spend a weekend in Philadelphia; she couldn't figure out how to arrange the child's travel in a way that made her comfortable.

The train? Too long a ride. And all the stops provided too many opportunities to get off at the wrong place and too many new (strange) people getting off and on. Conductors, when available, generally did not function like airline stewardesses. And what about the child's luggage?

The bus? An even longer ride, with all the stop-and-start drawbacks of the train. She settled on an expensive plane ride: short trip, no stops, and ensured attentiveness to her child by the airline personnel, both on and off the ground.

For younger children traveling alone, it is essential to work out a sensible, safe travel plan and make sure the young traveler understands it. Even more basic is to first be sure the youngster's heart is in the trip. This is the consensus of experienced parents, transportation personnel and child psychologists.

If it is a pleasure trip, it should be a journey of choice, and parents should look carefully at their child's reaction when the trip is proposed--especially if a parent or eager relative is doing the proposing.

Lots of "hemming and hawing" should be a red flag for parents, says one psychologist: "If a parent brings up the idea and then has to push it, it's better to wait until next year. After all, the child doesn't have to do it."

For young children who do have to travel and are wary of their first solo trip, it is especially important to go over the whole travel scenario in detail, from the airport metal detector to who is meeting them at the other end. The trip should not loom as a completely alien experience. And the younger the children, the more details they need.

How young is too young for a trip alone? Air, train and bus lines set the minimum age range for unaccompanied travel between 5 to 8 years old. Seven seems to be a favored year among parents of the veterans. One psychologist concurs: "A child ought to be able to read simple words like 'in, out'; 'women, men' (for the restrooms), and 'push, pull'. If a child negotiates first grade successfully, he or she can probably do a plane trip alone." Plane

The plane is the clear, first choice for parents of young travelers. And airline policies and personnel usually make matters the most comfortable for anxious parents.

U.S. airlines have fairly uniform regulations on unaccompanied travel (both domestic and international): A child must be at least 5 years old and must pay full fare. For ages 5 through 11, most airlines require what they call "advance arrangements" before a child can travel unaccompanied. This means a parent, or responsible adult, must bring the child to the airport and guarantee that an adult will meet the child at the destination. The parents or other adults must also complete airline forms that include the names, addresses and phone numbers of the dispatching and receiving adults.

Some airlines will not allow younger children to travel unaccompanied if a change of planes is involved. American Airlines, for example, permits solo travel that includes plane changes only for children 8 years old and up. Other airlines will ensure that personnel at the connection point help children find their way to the next boarding site; some require that a responsible adult also be present at the connection point (such as Aunt Jane in Chicago overseeing her nephew's connecting flights between Washington, D.C., and Grandma's home in Rapid City, S.D.). It is important to check out these varying regulations with the airlines when making a child's reservations.

All airlines are generally helpful with extras, like name tags, early boarding, even allowing parents to board with the child. One traveling child was thrilled to hear the flight captain ask over the plane intercom, "Are you ready to go, Susie?" Train

The train may be a good choice for slightly older children traveling closer to home. Most train policies on unaccompanied travel do not encourage widespread use of trains by youngsters.

Amtrak permits children age 8-12 to travel alone in the Northeast corridor (Washington, D.C., Philadelphia, New York, Boston) during daylight hours if they have reserved accommodations. This means children must have seats in the club car on a regular train, club or coach on a Metroliner.

Because long-distance trains have reserved seating only, Amtrak also allows 8 to 12-year-olds to travel unaccompanied during daylight hours outside the Northeast corridor, with two provisos: Children cannot change trains and the stations where they get on and get off the train must be staffed.

With the daylight rule, a child could, for example, take Washington, D.C.'s noon train south and arrive in Raleigh or Fayetteville, N. C., before dark.

Like the airlines, Amtrak charges full fare for children unaccompanied by an adult.

Amtrak also requires that a traveling child first be interviewed by station personnel; the stationmaster is supposed to make the final determination as to a child's readiness to travel. This process includes forms to be completed by the adult who brings the child to the station. Amtrak records the complete itinerary as well as the names, addresses and phone numbers of the adults on both ends of the trip (the child must be met). The adult also must sign a waiver absolving Amtrak of responsibility for a lost child.

Beyond the station, Amtrak does not ensure any attentiveness to the traveling child. An Amtrak spokesman noted there were on-board attendants in Northeast corridor trains and that color-coded tickets posted above the travelers' seats could help attendants make sure the children got off the train at the right place. But there are no guarantees of assistance. Bus

The bus is probably the least comfortable choice for parents of young travelers. There generally are few rules and no guarantees.

Both Greyhound and Trailways allow children age 5-11 to travel unaccompanied at half the adult fare. But neither bus line encourages children to go alone.

"We don't have the personnel to take care of children who come in unaccompanied," says a Trailways spokesperson.

Trailways handles the problem by spelling out all the risks for parents, then counting on the good will of the bus drivers to keep track of the children.

For parents who choose the bus, Trailways agents suggest:

* Pick an express bus or one with few stops. There are express runs from Washington, D.C., to Philadelphia and New York, for example.

* Put a name tag on your child with names, addresses and phone numbers of adults at both ends of the trip.

* Tell your child not to get off the bus until it gets to his or her stop. The child can use the restroom on the bus and bring a snack.

* Tell your child never to leave the bus station alone or with a stranger. If the child needs help, he or she should ask to see the terminal manager.