You'll need something quite wide with a small central transmission tunnel. For your budget it'll probably be quite a struggle, you really can't get much at that price which won't have high mileage on it. I'd suggest getting an older car, like a Honda FR-V, which is quite reliable. You might just squeeze two child seats and an adult in it. You could also try a Toyota Previa, but I'm not sure that you'll get the space you need. Hyundai Trajet is another one to look into.

Unfortunately, you can't guarantee reliability for that kind of money so make sure you ask a lot of questions about the car, check the service history and try to buy a Japanese car for reliability. What you don't want is to buy a cheap car and then so much needs done to it that the first service and MoT costs half of what you spent on the car. The best advice I can offer is to be smart when you buy, don't just go for the cheapest model you can find. Imports will be cheaper, but be careful because if you get something too obscure then parts might not be available if something goes wrong. Good luck with your search.