I just went through this, and know first of all that you are in my prayers. It’s never an easy thing.
If your parents are strong Presbyterians, be prepared with answers to the most common misconceptions of and Protestant protests against the Church: the role of the Pope, the fact that the Saints and Mary are not idols, an explanation of the Eucharist, why women can’t be priests, the necessity of Confession, etc. Also be prepared with your own personal reasons for entering, and absolutely be prepared to say, “Mom, Dad, I love you, but I have to walk away right now.”
In all of your interactions, no matter how they act towards you or react to the news, treat them with the utmost love and respect owed your parents, and ask your kids to do the same.
I agree with the others: tell them beforehand so they’re not blindsided in front of the kids, and have all discussions or arguments behind closed doors. I also like the idea of going with them to church, if it doesn’t interfere with Mass.
Praying for you! I hope it all goes well
Edit: I just want to clarify! If the discussions can be held in a rational manner, by all means have them with the kids as well! It is good for them to be able to rationalize their choice to join the Church. I guess I meant more arguments, since that’s how my family deals with it.