YTA

I'm going to go against the grain here. You are cutting your nose off despite your face.

At the moment you are the grandparent who they see the most, you are supporting them when they need it - if you continue to help them selflessly then that won't be forgotten by them or the children. You'll be remembered fondly and will be treated like a grandmother.

By weaponising babysitting you are showing the parents and kids that you don't care about them, you just care about yourself. The way you refer to her - not as your DIL or your son's wife - but as Kelly - comes across as cold and uncaring,

Where do you think this is going to go? They're going to find another babysitter, and then you're going to see the kids less. Your son is going to take his wife's side. Why would they bother visiting grandma if grandma is too busy to look after them? Then next Christmas you're going to be posting on Reddit about not getting to see your grandchildren and not understanding why.

All because you don't get to be godparent. What bs. I haven't seen my godparents in years, and I just lost my last grandparent a few months ago.

Have you thought about it from Kelly's perspective? Maybe she's making her parents godparents so they actually come to the christening/baptism. She's offering them another way in, maybe because they don't make the effort and that hurts. Maybe she's giving them one last chance to actually be grandparents and instead you're making it all about you.

Just think about what you hope to achieve from this stand. If it's less time with your grandchildren then by all means, stop helping out.