The rite recommends putting child catechumens together whenever possible:
255. For the celebrations proper to this form of Christian initiation, it is advantageous, as circumstances allow, to form a group of several children who are in the same situation, in order that by example they may help one another in their progress as catechumens.
On paper, it looks nice. Certainly it makes sense to celebrate the rites of the catechumenate period with larger groups of children. The Lenten period rites should probably be celebrated with adults, if possible.
Almost all American parishes I know of have just a trickle of youth catechumens at any one time. And there are at least three distinct age groups: elementary, tween, and adolescents–I can’t imagine effectively mixing these in catechesis.
