As pointed out, the children are not bound to fast or abstain, even on Fridays during Lent. In the US, the bishops have indicated that the Lenten nature of Fridays may be observed in a way other than fasting, such as almsgiving or prayer. If you were so inclined, you could make some commensurate charitable effort in lieu of abstinence from meat on any Friday outside Lent, if you were so inclined.
It would be gracious, in any case, to have something without meat available for those who choose to abstain, whether that is cheese pizza or something else, because your family may not be the only one in your circle that does this. The adults in your family can have just the cheese and there will be some for any other adults who abstain as their Friday penance or even any children who choose to abstain with their older relatives even though they are not bound to do so. Some, after all, do not look upon Friday penance as a burden, but as a way to be nearer to God. (I mean those who would miss it when the demands of charity require that they observe their penance in a way not of their choosing, because the alternative is to offend their hosts when duty does not demand it.) IMHO, leaving the traditional choice as a choice would be the hospitable thing to do.
I’m saying it falls under the Golden Rule: If someone were to make a non-meat alterative available at a children’s Friday party so that you could choose to abstain as you always do, I think you’d appreciate the thoughtfulness of the gesture. (Speaking of which: try to have veggies to munch on available, too, have cold water available in addition to caloric drinks, and take care not to push the cake on anyone. Someone may not only be abstaining, but on a diet!)
