Are your kids able to get F in tune without pulling the main tuning slide excessively? Start with that.
If they're playing wrong fundamentally, slides will be all out of wack. Usually kids play with too much tension or a tongue position that is too high.

Tune to Bb(2). If you're using 4 valves pull the 3rd valve slide and leave it. You can monkey with pulling slides or use alternate fingerings from there.