I fear that you might not be able to buy a CIN T-shirt in child sizes.
The BBC website indicates that there are 2 types of T-shirt available. The ones designed by Fearne Cotton are only available in M, L and 'Ladies fitted' sizes:
http://www.bbc.co.uk/pudsey/fundraising/buy.sh tml
The other design is only available in S, M, L and XL sizes:
http://www.bbcshop.com/icat/217&source=930
I hope that I'm wrong but, if the BBC don't sell them, I think that it's unlikely that you'll find what you're looking for. Sorry!
Chris