It looked like an ice cream giveaway, with hundreds of kids swarming across the Wolf Trap greenery, smiling, cavorting, making with joyful noises. It was in fact the annual International Children's Festival, a smorgasbord of arts, crafts and performances which may have drawn close to 20,000 children and parents by the time the three-day festivity ends this afternoon.

Along with the amateur folk and educational performing groups, there was also a very pleasant surprise in the appearance of the Contemporary Dancers of Winnipeg, a splendid chamber group which is one of Canada's oldest modern dance units.

With all the attention paid to the big-time acts that take place there, one tends to forget that Wolf Trap is a park as well as a performance site. The festival was a pointed reminder that the two functions can be combined happily. Sponsored by the Fairfax County Council of the Arts along with the Wolf Trap Foundation and the National Parks Service, the weekend bash kept 10 different staging areas busy throughout the daylight hours, with songs, dance, mime, magic, poetry, acrobatics, clowns, marching and of course, picnicking.

The serendipitous Winnipeg troupe, founded in 1964 by Philadelphia-born Rachel Browne, performed each afternoon and will today as well on the Filene Center stage. Tailoring the presentation to the youthful audience, the nine-member ensemble danced Sophie Maslow's 20-minute "Country Dance," and ebullient romp set to bluegrass and cajun music of infectious rhythmic verve.

Qhe company, just seen at Jacob's Pillow and now en route to the New York Dance Festival before returning home, deserves to be seen here at greater length and in a larger slice of their adventurous repertoire.

Elsewhere in the festival, variety was the watchword. A sampling of simultaneious activities Saturday afternoon turned up the Blue Sky Puppet Theater supervising a do-it-yourself session for wide-eyed tots and some furry animal puppets of indeterminate species: a quartet of prettily costumed teenagers from the Soviet embassy in a perky folk dance: a young man in short trousers crooning a number from "Annie," a local ballet troupe stepping lively in mesh stockings and bowler hats: and an afternoon finale which included toe-dancing, mime, impromptu sketching, fire-eating and singalong. If there were any dissatisfied customers, they weren't making themselves conspicuous.