The science centre is great, its featured on nina and the nuerons on cbeebies. It is about £8 to get in, although you can use clubcard vouchers. Definatley worth a visit.
We have taken ds who is 2 there twice now, and could easily spend the day there, lots of things to do/see/buttons to press.
There is Xscape, they have an indoor sky slope, mainly for skiers, but they do have sessions for wee ones where they can do slegding. Its a 10 min ride out of the city though.
Mueseum of transport is good, and free, and has a few bits from Balamory in, including Eddie MacCready's bus. Opposite here is the Kelvingrove museum, tis a bit stuffy IMO, but again its free. There is Kelvingrove park too which is lovely, they sometimes have fairs on and such like
The peoples palace on Glasgow Green is nice, there is a lovely cafe in the glass house, a huge park, gardens and mueseum. There is also a brewery opposite that I think you can have a look around.
If its a nice day the city itself is nice to walk around, there are open top bus tours, which are a good way to see lots of things.