The advantage of in-ear phones is that they cut external noise so that the volume doesn't need to be very high to get excellent sound quality. You do need to make sure that you've got a good seal with the ear, and this is dependent on which size earbud you need. So take a bit of time getting that fit right.
Sound quality compares well, I think, to other in-ear phones at this sort of price point and is significantly better than iPod/iPhone earbuds (note that they also 'leak' sound a great deal less than iPod earbuds do). In terms of volume, I thought they were plenty loud enough attached to an iPod; I found myself listening at 80-90% of maximum volume. So as a parent I'd be reassured that my child wasn't damaging either their hearing or their musical enjoyment.
These also come with small and extra-small buds, so as well as being good for older children, they would be very suitable for adults who find that regular in-ear phones tend to be too large.
A star off because although these are designed and styled for children, they're no more sturdy and durable than adult in-ear phones; which is to say 'not very'. UE say that they're suitable for 6+. They're obviously fine for a teenager who will be careful with them, but I wouldn't give them to a smaller child who might be harder on their stuff. As with all in-ear phones you should discourage your child from sharing earbuds and keep the tips clean.
These headphones are styled much like my very grownup Ultimate Ears phones, so my daughter (13) assumed that the UE logo on mine was also a 'loud enough' logo. She's very impressed by the styling and the sound quality, and says they're plenty loud enough.