http://www.tuesdayschildagency.co.uk/
Try this, yummymum. This's a legitimate modelling agency, who have children who've been in TV adverts on their books.
Can I just say, that a child doesn't have to be especially beautiful to look at, but has to have that certain "something" which can be captured on camera. Modelling agencies also look for very well-behaved children, as waits between shoots can be very long-winded. They prefer parents who aren't pushy as well, and you have to be available at any given time to chaperone your child to auditions and photo shoots. Best of luck. x