It’s always best to buy from a specialist. Some camera shops have a good range, too, but they rarely know much about what you need for birdwatching.
High street chain stores are often even less helpful, but they may stock exactly what you are looking for as a birdwatcher – you just have to know what that is before you go in.
The RSPB online shop also sells a pair of children's binoculars.
View this page online at http://www.rspb.org.uk/youth/discover/environment/binoculars/buy.asp