10 seems like a good number, but make sure you can afford that many crickets per week. That's like...30 a day if they're juveniles. Slate will work as long as there are no sharp edges...sand em down? I would suggest a filter for the water as well. I would use eco-earth over sand also, it's frog safe and just as good..Only downside is it gets in the water, but some leaf litter on top can cut down on that. You could put some small guppy feeder fish in there for the frogs to snack on as well.