A friend recently posted a link from this blog entitled "How to Make an Inexpensive, Homemade Mass Kit for your Kids." My initial reaction was discomfort. On reflection and some discussion with people, I came to this thought:
If children want to do pretend baptisms, liturgical services, etc. that's wonderful and a very natural thing to do (my children have done it as I expect have many others). It's when we purchase things or prepare them to mimic holy things (spray painting things or putting crosses on them) that I have concerns. The diskos can easily become a peanut butter sandwich plate or frisbee, the chalice can hold cheerios, etc. I'm sure the immediate response some parents will have is "This just needs supervision," but my experience as a parent has been one where successful sequestration of toys has been spotty. All toys end up in the bathroom eventually. What is the freak-out level of these purpose-built toys in there?
If you want to open up the idea of the priesthood/monasticism to your child, the domestic church offers lots of other possibilities (making prosphora and other festal foods) as does the parish itself (prompting them to be altar servers, preparing flowers for feast days, or singing in the choir).