Ramshorns are fine to keep in aquaria and are about as hardy as they come!
You can get away with not having a filter if you are only keeping them for a few days; any longer than a couple of weeks and it would be best to have a filter in there (only a tiny one) for the snail's benefit. Children can handle the snails, but only whilst the snails are in the water. It is probably best to use an ordinary garden snail (or a giant african land snail for the size impact!) for this purpose.
If you are keeping them for a few days then there is no real need to feed them anything special; if you just put some pond week in with them from the pond that you get the snails from then they will just munch on that. Again, for longer periods of time it is best to feed them small amounts of sinking catfish pellets.
I woudent fill the tank with tap water, you can just use water from the same pond that the snails are from. Or, if you are buying your ramshorns, get some water from your parent's pond to fill the tank with. This will not have any chlorine in it which will harm snails.
You can release them into your parent's pond if you like, they will be quite happy there.