http://www.beehiveillustration.com/
Children's book illustrators agency.
http://www.illustrationweb.com/
General illustration agency, but with children's illustration section.
http://www.thebrightagency.com/
Able to create own accounts and submit images, it seems to be an open agency.
http://www.organisart.co.uk/
A general agency with a strong focus on children's illustration as much as the other forms.