Hi
Depends on the tastes and interests of the children, but if the weather is good, they'll not want to move far from the beach or Hotel pool during the day. Western Water Park and Aqualand are both open if they prefer a day's water park, as is Marineland in Portals, although I've heard a couple of negative views about that lately. There's a rumour they are looking to move it to Magalluf, so there's a suggestion that they are not spending much on updating and improving it.
A bus ride will take you to Santa Ponça for a day out, or to Andraxt Market on a Wednesday morning for the Market. There is a big Car Boot / Flea Market in Magalluf itself on a Saturday morning which is quite interesting, although there is a lot of worthless junk there.
You might want to get a bus into Palma and see the Cathederal, and walk around the shops, or even get the train to Soller.
You will never be short of things to do at night - depending on the Hotel, there may be games / entertainment organised by the Hotel itself, but there are hundreds of bars in Magalluf and Palma Nova that cater for all tastes and ages - don't be put off by people who "pigeon-hole" Magalluf as a resort full of drunken youths - you'll find drunken youths (and people old enough to know better) at every resort in the world - the thing about Magalluf is that it has "lively" areas where those who want it go, and there are areas of the resort which are quiet, and cater more for Children.
Pirates is a good show, although it's not cheap (don't book through the rep, as Pirates are offering a 20% discount at the moment - and you won't need transport - you can walk there in 5 minutes) and you can do yourself a favour by going to Woolworths or Wilko's (depending on where you live) and buy a cheap Pirates kit (sword, eye patch and bandana) before you come out for about £1.99 (or less) and save the 10 / 12 euros they charge for the same at the end of the show! The food's OK, and the drinks are limited in choice, but free.
Most of the trips and excursions that the reps offer you, you can book yourself cheaper - and if you're kids make friends they'll be happy to spend time (sorry - hang out) with them most of the time.
Our son is 10 (going on 19) and he obviously lives out here, and is very street wise - we barely ever see him - Fishing, swimming, it's an ideal place for kids that age.
They'll have a great time.
Cheers
Adam