If the children are few, I tend to do a master class longer than the act. Because children really like it. I cut the number down to 10 minutes, and then we just blow bubbles with them.
If an artist has been around for a long time, especially at children's parties, he/she can diversify, to arrange a master class in the form of some kind of game or competition.
E.g. summon a couple of children, split the rest of the audience into teams, one supporting Pete, the other rooting for Mary. Now dip the wand, and command: those who will blow the most bubbles in 20 seconds wins; count to 20. Children count the bubbles aloud; the artist checks at the timer.
Then ask the children how many bubbles they made. The winners will be applauded to. It's a good idea to give prizes to everyone. If you add such contests to your script, you will extend your gig up to an hour. Children like it, if you want, you can do so.