The period following Communion and before the dismissal is a time for meditation. This is also a time when announcements are usually given and laypeople are allowed to give remarks on a topic that has been approved by the pastor. It doesn’t seem out of line then to allow the VBS participants to sing a song or two during that period, assuming that the songs are appropriate for Mass.
The only difficulty I can foresee is the applause factor. Naturally, unless they have been previously instructed otherwise, the congregants are going to assume that they should applaud the children’s presentation. Applause that is meant to offer a critique of a performance, as if this were a performance offered to an “audience” rather than a presentation offered up as worship to God, is *not *appropriate at Mass. If the children are prepared to understand that they will not be applauded and the congregation has been instructed in the past on the expected protocol at Mass, then the applause factor won’t be a problem.