Airport viewing platform is only worth it on a sunny day. You have to pay per HR for the parking and there is not much to do other than watch planes land!
Trafford Centr is free parking and has a lot in the entertainment part, bowling cinema, soft play, amusements as well as legoland and sea life.
If planning on going to Legoland and sea life - check Tesco club card for free tickets as well as cereal boxes etc. IMO very expensive full price but great fun if it were cheaper.
Opposite Trafford Centre is Chill Factor E and play factor e. Free parking again - loads of great activities not just skiing. Play factor e is pricey for soft play but worth it. Climbing wall at chill factor as well.
On same complex as chill factor is David Lloyd leisure and DW fitness. Blag a day pass or trial and make use of the facilities. DL has a small soft play area, kids pool, outdoor swimming and nice-ish cafe.
MOSI is great, small train ride is a firm favourite with my DS.
Check out Lowry Theatre in Salford Quays as they often have cheap shows on aimed at kids. BBC and ITV studios also based here and have tours/various activities on especially during school holidays. It's nice to go anyway for a walk around the quayside and over the bridge - especially if you have kids that need space to run, gets cold though. Good restaurants as well.
Hope you have a fab time!!