Definitely a win. Or at least over one of them and you worried the other. I would put it down to their lack of Maturity (Or lack of Mature years).

Problem with youngsters is that they do not know how to pace themselves. They go flat out on the easy bits, like on the flat, and then have no stamina for the hard bits.

If I was you- I would get them out on more hills. Forget the easy bits and show them how to climb.