This is the clearest explanation I have seen of the interest/tax issue. But I do know people who've been done by the revenue for this, so you have to be very clear that the money did not come from a parent.
If a child generates more than £100 interest in the course of the year from money specifically given by each parent, this income is taxed at that parents' tax rate.
In practical terms this means you could put up to £1,850 in the 5.7% top paying children's account, and it wouldn't be taxed, as that would generate around £105. Just to clarify, this doesn't mean £1,850 every year; it's the interest generated from all cash given in this and previous years.
Yet these rules only apply to parents, not grandparents, aunties, uncles or friends ? they may all give your children as much as they like and, providing it's a genuine gift, it counts as the child's money without a £100 limit.