The weather's going to be good tomorrow, so get them outside. Suggest Dunham Massey, Wythenshawe Farm, Stockley Working Farm (or even Lyme Park) in South Manchester; I don't frequent North Manchester with kids, but have been to Heaton Park once with 4-year old and that was good.
Another vote for Dunham Massey - our ds's (3&1) love it - ducks to feed, a working water wheel, deer poos (and occasionally deer) to examine - and a fab shop and restaurant for Mummies! The restaurant has a kids' section so is also perfect for them!
It's one of our favourite places to go when the weather's nice!
Marthamoo! Styal's fab, but not brilliant if you've got a pushchair - tons of steps inside the mill. I know they lend out baby backpack carrier thingies, but our ds2 was a bit of a weight to carry around! But the playground's fab and outdoors bits.
Another vote also for Bruntwood park - I love it 'cos it's got separate play area sections, so you can waste/spend loads of time just going from one bit to the other and the kids get excited each time!!
Good point - I've an 8 year old and a 3 year old so they just ran round the mill like lunatics ("sorry, nice NT volunteer - I'm sure your working loom is very interesting but I have to gooooooooo....") - it isn't at all pushchair accessible really.
My two love the spinning turny handle thing and the climbing wall in the play area though.
Dunham is great - take a bike for the 5 yr old. Bruntwood also good but we don't know where you are!!
Wythenshawe park is free and has a little farm, a rainforest junge thing which is great and quite a nice play area. Although last time we noticed that everyone there with a dog had a nasty pit bull type thing NOT on a lead - bit scarey! AND my friends ds1 fell on the grass and got literally covered in dog poo - lesson of the story - stay off the fields and remain in play area!!