Star, best idea I can come up with at short notice is: how about Bristol? Not too far from you, so more holiday time!
We went recently and had a great time. They have a great big new sciencey/interactive place called '@ Bristol'. Perfect for your twos ages. Loads to see and do. There is an IMAX cinema too, we saw a great 25 min film about dolphins, it was really WOW in there. 6 year old loved it. There is also a rainforest area. Pizza restaurant too, fun fountains outside.
Lots of hotels have offers including Thistle Bristol (£29.50 per adult per room min 2 nights) and children free, includes breakfast, they have pool etc for kids.
Or is you want some vegging out space there are some serviced apartments in the centre. (Howard Johnson, they have a website) One bedroomed £65 per night. Based on 2 adults, 2 kids. Compared to a hotel you'd have more space (but no pool).
Great website called visitbristol.co.uk. You can book hotel/apartments at Bristol Tourist info office on 0117 946 2222.
Bristol Hippodrome theatre are currently doing Beauty and the Beast, which is AMAZING, again 4 and 7 year olds would love it. Ticketmaster have 4 seats available quite high up, but binoculars are half the fun!!! £52.00 for all. Could try at box office on day for returns.
Bristol also has zoo, etc. Plenty for a fun weekend I think whatever the weather. Great shops too, as if you would be allowed!
Let us know if you hate the thought of Bristol, and I'll think again!