Children who have received their First Holy Communion belong at Mass with their families. To remove them from the Mass does them a great disservice, and is incredibly disruptive.
My parish has the children’s liturgy at one Mass a weekend. They all parade out, and when it is through, the children all parade back in (often quite noisily despite the leader’s continued shushing). One time they came back in during the consecration, right in front of the sanctuary, wandering about trying to find their respective parents.
I know people mean well, but removing part of the congregation to another room (or in my case, another building) is an act of separation and distinction when we should be celebrating the holy sacrifice of the Mass as one body, and this is particularly true of those who have already received their Holy Communion. There is no reason to separate them. If people are worried that they won’t understand the readings, provide the simplified readings in the bulletin and expect the parents to go over it with them.
Also, I suspect that many of the adult leaders have little or no special training to explain the readings. I know this true in my parish. The volunteers are simply parents who read out of the children’s lectionary and then do a craft with the children.