Oh yes- do not eat there -thats a vom not an envy! - there are dozens of places around Brindley Place to eat... and if you're going to Think Tank too- you have the whole of the city to choose from between Sealife and Thinktank.
There is a bit of interactive stuff, but thinktank is better for that really- though I haven't taken my own children to thinktank yet, as too young really. It is v expensive if you're paying to go in, and the area for young children is smallish.
If you want a 2 for 1 adult ticket for sealife centre, CAT me and I'll post you one, your 4 yo has to pay child admission though. (we haven't been since dd was 3, a bit annoying that 3yo have to pay when you never know if they're actually going to look at anything )
Hotels... well, there are plenty of chain hotels close by- don't think they're anything spectacular, but at least you sort of know what you're getting. Check websites for special offers, though if you're coming midweek less likely of offers as the hotels all cater to business in that area. We live quite close so don't stay over.
In the same area- Ikon gallery is quite good, and usually has something to interest young people; Bham Museum and Art Gallery in the centre is quite good, and free. Usually has childrens activities in holiday times (eg- half term v soon). The central library often has story telling stuff on too, its pretty close. (they all have websites)
Are you driving? There is a nature centre at Cannon Hill Park which is quite nice for a few hours for small ones- its about 1 mile south of city centre- it is on a major bus route too though, but depends how easily your DS tires. Its part of a massive park though, so if you actually want to wear him out....