If your kids are old enough to need a work bench, they would be just fine using a regular sized bench.
Jack
Jack
How Do I Build A Work Bench For Children?
Here is a link from where you can get step by step plans for a workbench for your children:
www.hammerzone.com/archives/workshop/bench/below20.html
www.hammerzone.com/archives/workshop/bench/below20.html