this really should have taken place after Mass, not during the Homily.
Better before and not after. Doing it before allows the priest (I presume that a priest was present as the OP said it was a mass) to refer to the play and use it as material for the homily. If the priest is not good with children, he is allowed to delegate to a layperson.
Generally, I make a distinction between children’s participation and the ministry to children. I assume that this is not a mass with only children and adult ministers to children but a mass where the general congregation is present. The guiding document is the
Directory for Masses with Children.
The priest’s primary responsibility int he Gospel of the Word is to bring the congregation to a deeper state of spirituality during the hour at mass to allow us to commune with God in the Eucharistic elements. The idea of the homily is that by the time we reach communion after reflection on the message for the day (often, there will be a theme/image /etc that the priest prepare the liturgy around). Most of the time, the priest will do so directly with a sermon.
Alternatively, the priest could use other more novel ways to bring the message across. One way that I have participated in as a parish catechist, is having a dialogue between the priest (and myself there sometimes) with the children. This particular priest that I worked with was careful to work the dialogue in such a way that it is also directed at the adults present.
The priest in this case could allow the play (but I would prefer that it be done before the mass starts) and the incorporate some illuminating points in the play into his homily. This would be better for the children’s participation if it is explained to them that they are not merely doing a play but being part of Father’s message in his homily.
Children generally do not make good readers (there are exceptions of course) and we have all atttended mass with poor adult readers which leaves us either lost, confused or distracted. As a result, it is best not to have children reading at the general mass, unless the entire congregation is OK to be there to smile at ‘cute’ or ‘sweet’ children stuff instead of spiritual sustenance of which understanding the Gospel & homily is a fundamental pre-requisite.
Refer
here if you have any question on church rules regarding Children’s Liturgy.