Decathlon. Get to a store if you can, as they have the worst website EVER, but their own brand gear is very good quality.
DS1 is 3 and wears the size 5 stuff though (admittedly with a bit of growing room) but he is on 90th centile for height or thereabouts.
I have the trousers and the one-up-from-most-basic Cagoule, very pleased with both of them. The trousers have Velcro tabs at the ankle so you can get a pretty tight seal - dry feet after over an hour of stomping in puddles the other day