One of the most important realities when conducting focus groups with children under the age of 10 is that they are very much influenced by their peers' opinions.  Having a discussion about a product with a group of 6-year old children can go one way while actual interest in the product can give you completely different results.  In comparison, you usually have a good idea about whether or not a group of adults are going to be interested in your product or concept by the time you show it. Not so with kids.  They will talk up a storm about how much they like something only to have it fall to the bottom of their list when choosing favorites.  Not to mention that girls are often more agreeable than boys and hesitate volunteering what's wrong with something. 

Two important steps you can take to ensure the accuracy of the information you're getting:

1)  Let kids know that they won't hurt your feelings if they don't like something because it's not yours and you didn't make it.  Give them the permission to speak freely.  This isn't a guarantee, however.  You still need to follow up with probing questions that you intuitively think will get at the problem.

2)  Take a blind vote of your product in an array of competitive products.  Once the products have been discussed, have each child vote on his or her favorites one at a time while the other children in the room have their eyes closed. Sometimes it's more effective to do this earlier in the discussion so you can see what's really going on and then include those anchors in your discussion.