I think £5 a day is fair, and I personally wouldn't pay any less. Doing someone else's animals always takes longer than your own and it is a responsibility - they have to be there for the whole week. It can seem like a lot for jobs that should only take 15min, but given the responsibility I think it's fair.
A lot depends on whether you are likely to want the teenager to do them again and whether you have difficulty finding people to do your chickens. I always err on the generous side when paying people to do my animals on the basis that they should be keen to work for me again.