Hi Mom, and welcome to Cruise Chat.
I haven't sailed on MSC, so I can't speak about the kids' activities. However, there's usually no down side to signing up early for excursions. If there is something you know you want to do, you avoid the possibility of it being sold out by the time you sign up on the ship. Also, you avoid the sign-up process onboard.
Check with your travel agent or with MSC first, to get their cancellation policy regarding advanced sign up for shore excursions. Many cruise lines allow you to cancel up to 24 hours prior to the departure of the excursion, but I don't know if that's MSC's policy as well.