Of the three resorts Playa Andaluza offers the most for kids, particularly young ones. It's a compact resort so there isn't far to go, there is a dedicated kids pool with slide and splash pool then there's a sports pool for the older kids where they usually set up a net for water polo etc then there's the football court, tennis court, climbing wall, mini golf, table tennis, giant chess etc. Marbella offers similar although the kids pool is indoors (there is an outside splash pool but not quite as good as Andaluza) and the other activities are more spaced out. Son Antem doesn't offer a separate kids pool however the real benefit, in our opinion, is the 'private' pools shared between the villas. We regularly have these pools to ourselves or at least shared with some sunbathers so in effect they become your own private kids pool and if you're lucky enough to have a villa adjacent to the pool it's great to be able to prepare a barbecue whilst being able to keep an eye on the kids. There is a main pool in the central complex which is often full of children and it's surrounded by a large sand area with a volleyball net, a small play park and a football court. All three resorts have a good kids club.
Marbella has the better beach over Andaluza however for the most part, people tend to prefer the pools and lounging around the resort rather than being on the beach. There are a number of hawkers hanging around outside (particularly Marbella) trying to sell knocked off handbags and the like. Besides, there are only a few months where the sea is warm enough to be enjoyable.
At Son Antem, El Arenal is only 15 minutes away and has a long stretch of sandy beach. It is an area favoured mainly by Germans and has a lot of amenities, hotels etc. We prefer Es Trenc beach which is a 30 minute drive away which whilst it isn't a simple five minute walk across the resort it is fine if you're planning to spend a day at the beach. There's also Aqualand (a water park) in El Arenal and one in Torremolinos (30 minutes from Marbella or 50 minutes from Andaluza).
If I were to decide between Marbella and Andaluza it would depend on the ages of the children. Ours are 13 and 11 and at this age they prefer Marbella although it's a very tight vote. I would say Andaluza is probably the better choice for younger children although both are excellent. I would be happy with either choice if it came up for exchange.