Not sure about much for a 9mo TBH, although in two of the Under 5 areas there are little soft areas for tinies. Lots to look at - colours, things going round, etc.
Dd is 3y4m and she loves it. This was her seocnd visit.
Definitely go onto the website link I did for the 2 for 1 ticket - then your trip will be much cheaper anyway, although parking is £2 for up to 5 hours at the pay and display next door.
And I would recommend a picnic. the cafe was buy and the food is basic soemthing & chips, or sandwich boxes. Picnics can be eaten outside, or in the train carriage in the gardems (DD loved that )
DD's favourites from today:
* the M&S shop
* the kitchen in the house
* the garage
* the sound garden (or whatever it is called) - she loved painting the butterflies on the computer screen and seeing them fly onto the wall
* Me and my body
* seeing Gordon the Garden Gnome
* the maze outside