tooexcitedtospeak, it's very hard to actually say which agency is best because everyone has a differant experiances, all i will say is do your research and don't accept the first one that says yes, check out the jobs that each agency has done and then maybe you can suss out if that agency is more suited to yours childs age group. here are list of agencies that are known on this site to be repertable, low start up and gets lots of work
mot junior - www.motmodels.co.uk
bruce & brown - www.bruceandbrown.com
kids london - www.kidslondonltd.com
urban angels - www.urbanangelsagency.com
these four agencies are by far the top four agencies with no or very minimal start up costs. i have seen children from all of these agencies at castings that we have been to.
also have seen kids from www.bizzykidz.com
elisabeth smiths - www.elisabethsmith.com
norrie carr - www.norriecarr.co.uk
these two agencies are very well known and very well established with in the industry but do have a much higher start up cost.
make sure you send off a really good clear head shot photo a good photo can make the differance between yes or no. good luck