I can see where you are coming from, OP, and I would probably have similar thoughts in your position.
However, in addition to your dil's wanting a 'break', I think there might be another argument in favour of not changing the arrangement over the holidays -routine. Particularly with such a young child, it might be less traumatic overall for him to stick to the usual routine, rather than having to re-adapt in September.
That said, like you, I find it very surprising that his mum doesn't want any extra time at all with him over summer!