Pros and cons -
Pro - We've had young horses before and they can be perfectly good if you know what you're doing. We bought a 3yo Welsh A palomino gelding (for 1,000 pounds)and broke him in ourselves and turned him into a very nice showing/riding pony. Buying young meant that we didn't get any vices. However we do know what we're doing and I couldn't recommend this route to everybody.
Cons -
a. Would definitely not buy him until he is gelded
b. 11.2 is not very big so you have to factor in that your children will have outgrown him by the time they are around 7/8.
c. I don't really understand the term cob in this context. Usually this is a word to describe a Welsh Sec D but yours sounds too small for this. I presume he is just a part bred with a bulky body and cresty neck. Does he have any passport / papers at all?
d. As you are posting this in January do we presume he is now 4 rather than 2 turning 3 - the latter would worry me as too young for a riding pony. Also he needs to be 4 if you are going to do things with him such as pony club, shows, gymkhanas etc.
e. Hopefully you are not buying on the basis of price. Buying the pony is the cheapest bit and I'd always recommend spending as much as you can afford on a better, more proven pony as they all cost the same to keep! We sold a top little man last year who could show at county level, go on or off the lead rein and could jump at pony club level. We only got 1,200 for him as it is a buyers market so you should be aware that there are proven horses out there for not much more money.