I'll start with pyjamas -
I got these from Vaenait Baby who sell through Amazon.co.uk. They cost €8 to €10 per pair and come in a multitude of colours and patterns.
They're posted from South Korea and I'd bet my bottom dollar they're made in a Gap or Old Navy factory - the shape is exactly the same - but with different labels. And a different price. Great quality, they wash well and the sizing is pretty small. My tall four year old wears age 8-9. (They are big on him - I could have gone down a size.)
Next is Boden, a saviour for mothers of boys, especially families of boys as everything lasts well and can be handed down from boy to boy.
Boden tops, pyjamas, and tracksuit bottoms are staples here. There's regular discounts but going online just after Christmas works well for me. Long sleeved t-shirty tops like those above are always well reduced.
I learnt about the Mango Outlet site on All Worn Out. Thery have great deals for kids and adults. I scored these shirts. Soft and lovely and sort of jackety with those patch pockets on the front, I love them. And more importantly, they are being worn.
Lastly, underpants. I ordered a set from Gap once for my youngest, really just to get up to €50 so I could get the free shipping, and I was a convert. They are softest, cosiest, best quality around. And there's 30% off everything in Gap at the moment (until Monday night)with a Grazia magazine code.