depends on thier age, if we are talking young kids, like around that 8yrold area, then bike time and playtime. ride the bike, be kids. they will get thier plyos by jumping and swinging from playground equiptment and things like that.
i thay are hitting that pre teen, 12 year old mark, then it isn't crazy to start thinking about doing some off the track wrk.
now with that being said, track skills are essential, at any age. off the track work is there to suppliment your racing, not the other way around. lots of people get hung up with gym training (i'm guilty also), but the work you put in on the track is most important.