Joker Tube and Strat-O-sphere are both the same effect, and I highly recomend getting one of them. Joker Tube is availible from this site.
For the age group that you speak of, magic coloring book is great with dissapearing crayons. But older kids will work out how magic coloring book is done.
Dice Box is also a great effect that kids love.
I would also recomend you getting hippety hop rabbits and peanut butter and jelly illusion.
If you've got the space, the time and indeed you want one, buy a rabbit, and rabbit mirror box. Kids will remember you for a long long time if you do this, and adults will too.
Remember in a kids show, the thing that really matters is you entertain your audience.
Hope this helps.
PS. You may wan't to check out the kids showforum, if you havn't done so already.