I want to teach the kids to prusik i have a petzl simba and I'd like guidance of how best to get the jammers onto the harness, they need to be lower and keep them central or maybe i should make a harness from a small sling?

It's not really very practical using a Simba. You'd be much better off making a low C of G harness out of a fixed length webbing sling (if you have one that is the correct size) or make one up with a tape knot out of a length of suitable webbing.

Mark

"texas kick" would work.k x

Small kids have an amazing ability to fall or squirm out of harnesses of any sort. They need a solid chest harness as they have no hips.