We have two boys, and when we got our Mass set, I made it clear that it was under my jurisdiction, and not theirs, because although a Mass set is not a sacramental, it is also not just some plaything among playthings. They were not to use those things that were in any way irreverent or jealous or mean with each other, and if they did, they weren’t going to be allowed to play with the set. OP, you may want to make that clarification: you bought the set for your son to play with, but it is yours. When you’re a grandma, after all, don’t you still intend to have it in good condition for the grandchildren?
If you son wants to be a priest, he had better learn how to deal with female parishioners, because a lot of his sacristans will be women. He needs to share the family Mass set with his sister, and let her help him set up and be his parishioner. I would also encourage you to consider training the sister to be his altar server. There is a lot to do in that role! Since she can’t be the priest, he’s not going to able to be the altar server, not unless they can scare up another boy to play with. She can also be the choir and every other role that the laity can take on. She’s going to be one busy girl. They ought to both take care of the set, however, because priests have to be ready to be their own sacristan.
A friend of mine has a son who is an excellent altar server. She had not intended to allow his younger sister to be an altar server, but then she rethought this after considering that a seminarian who cannot work with women is not ever going to be ordained in our diocese. They don’t ordain guys who think being a priest is about being better than the laity. For that reason if no other, the rule in our parish is that we do have female altar servers and do not allow boys to serve who refuse serve with girls or to have a female as the lead server. Since any bishop can come along and decide that his priests will accept female altar servers, it will do your son well to be obedient when the command from above is that female servers are to be allowed.
You and your daughter can also go on the internet and see if there is a congregation of sisters who have a habit that she’d like to be able to have for pretend. If you have any ability with a sewing machine, most will be easy to make. (I would hope for your sake, however, that your son never gets it in his head that he wants to be a member of the Swiss Guard!

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