We tried these walkie-talkies out and they worked really well. The kids like them - they think they're cool, like spy stuff. My wife loved the fact that even if the kids were out of sight she could speak to them and the kids loved the freedom of being able to wander a little further.
They're made by Cobra who are a US firm and they're sold fairly extensively. I think Costco do them and I've seen similar types in John Lewis and even Tesco.
The best price I've found for them so far is from a site called walkie-talkie-2-way-radio.co.uk which has a good range of Cobra radios in stock.