But you could buy pants the same colour as the tweenies and say 'these are your milo pants', IYSWIM
I have just cracked it (that'll probably jinx it) with my boy and we make a big thing about wearing 'big boy's pants' which are an assortment of any old colour, but the important thing is they are different from nappies and special and just for him.
We got loads of different colours from Asda - and go a couple of sizes up because they find the snug ones hard to pull up and down by themselves.
You could try ASDA. I think they have got some tweenies-stuff. I bought Tweenies pyjamas from them. Be careful though. My DS doesn't want to take his Tweenies pj's off. He is always wearing them now.....
thanks for your suggestions everyone. Fairymum, you have a point there - I may just be making trouble for myself! I have bought some plain pants already so maybe I should start with those and take her shopping for some tweenies pants as a reward for doing well. I may well be posting on the behaviour/development board later.