Eight year-old Barbie Ewen sat high in the saddle, enjoying her ride atop a chestnut brown Welsh Mountain horse. But she couldn't go very fast because she needed an attendant on either side to hold her on the horse.

Barbie suffers from multiple sulfatase deficiency, a rare neurological disease that affects her growth, sight and speech.

"She can't do many things. Her balance is really bad," said Kathy Ewen, 36, Barbie's mother, a nurse in Montgomery County. "But horseback riding is something she can do. It's something she really enjoys, and it helps her back muscles."

Barbie, her parents and two sisters were among 400 mentally disabled children, their parents and friends who spent Saturday at Rock Creek's Therapeutic Horse Center at the first annual "Let's Play to Grow Day".

The Joseph P. Kennedy Foundation has organized "Let's Play to Grow" clubs in Washington and across the country to bring together families of mentally disabled children in support groups, but this was the first year the program sponsored a picnic in addition to its other activities.

"We try to bring families closer together through play activities," said Mike Pickett, 31, coordinator of the national "Let's Play to Grow" program, which has approximately 200 clubs including 6,000 families with disabled children throughout the country.

While Barbie Ewen enjoyed her horseback ride, other children tried out a small trampoline. Parents helped some to bounce up and down.

Clowns helped children understand how to fashion long narrow balloons into dogs. Other children clutched their crayons and earnestly drew designs on kites they had made while others played football and volleyball.

"They get to experience something different and something that other normal children experience -- being with friends, the excitement, food, music," said Barbie's father John Ewen, 37, a Defense Department analyst. "People don't look away here. They acknowledge themselves. People can come here and meet with other parents who share a common experience and who understand."

His wife added, "You can feel very uncomfortable in the community. There are many activities the handicapped can't participate in. It's good for our daughter to be included because oftentimes she feels left out."

The "Let's Play To Grow" clubs bring together all members of a handicapped child's family to participate in simple physical and sensory activities such as dancing, throwing a ball and drawing, said Shirley Rees, 50, coordinator of the four clubs in the District area and their 80 member families.

The program, she said, believes it is important that all family members are included in the special activities designed for the disabled.

"It's not easy for the normal kids because the emphasis in many families is often placed on the handicapped kid. Well, the normal ones need help too," Rees said.

On the playing field, Joseph Stevens, 68, encouraged his grandson Carroll, 19, who is retarded and hard of hearing, to join a group of young men playing football with former Washington Redskin player Roy Jefferson. Carroll caught a pass from Jefferson and quickly threw the ball to another player downfield.

"It gives the child a feeling of closeness and togetherness, and it makes the parents get to know each other," said Stevens, a retired Federal Energy Department clerk, as he watched his grandson.

"I try to get him into football or basketball. . . . I hope that one day he will join the groups more and let himself have fun," added Stevens, who has cared for Carroll since his grandson was 3 years old.

Helping a disabled child is a cooperative effort on the part of the family, said Mary Belisle, 33, a homemaker from Warrenton, Va., whose 2-year-old son Benjamin suffers from a mild case of cerebral palsy and is hard of hearing. Both her daughters Dorothy, 7, and Crissy, 5, who are normal children, are learning sign language so they can talk with their little brother.

"They have accepted the fact that he is slow," said Mary Belisle, who herself is partially deaf. "They are seeing that this is normal."

Looking over the groups of children and parents, she added, "To see other people with a handicapped child helped me realize I'm not the only one. It helped me cope."