Sports Direct always have a sale on and I will admit I went in to look at stuff for me but now have made 3 trips to buy nieces and nephews lots of cute clothes. Admittedly not worth the full price they sell them for but definitely worth it for £2 or £3 a go.
They do online as well but you have to spend over £50 to get free delivery, better to go instore if you have one locally.
Barbie and My little pony childrenwear £2-£5
Marvel shorts, tshirts, and vests £3 each
Age rangers 6 mths to 9 years