That would be setting a bad example and teaching them something that they may take into adulthood erroneously.
Children do not control whether they go to Mass or not. They are not culpable for mortal sin if their parents did not take them.
Children are capable of committing serious sins at their level-- lieing, cheating, being mean to others, stealing, etc. If they have serious sin, they should confess it. They should be encouraged to reconcile through the sacrament before approaching Holy Communion.
One doesn’t need to have anything memorized. You can print out a simple order of confession of the internet and they can take it into the confessional with them and tell the priest this is only their 2nd or 3rd or whatever confession and that they haven’t been to church in a long time.
Best to start them out on the right foot.