If they're racing, IMO wiseco is the best choice. You have to remember when using a forged piston to warm the bike up properly before wringing on it though, and clearance is important so check the fit. Remember, when going aftermarket with the piston it's usually better to go with the piston manufacturers clearance rather than the OEM specification.
A cast piston can also work just fine and cost a bit less, but there too it's important that the clearance is within spec. Too much slop with a cast piston and you risk shattering a piston skirt, which can cause a great big expensive mess. If you have a shop manual, changing the top end will be no sweat. You'll be surprised how quick and easy it is.
Barnett, EBC, and Hinson all make quality clutch kits. There are also cheaper options that may or may not get your kids through a season, depending on how they ride.
Best of luck!!