Do they ignore your boundaries for your children (eg. if you say it makes your life hard and your children miserable the next day if they miss their midday nap or go to bed much later than usual do they plan things which make it impossible to get the children to nap and into bed roughly as normal and make it hard to say no without a big argument and upset children?) and overrule you, ridicule you saying no in front of your children, call you things akin to a "fun sponge", come over uninvited at bedtime and rile the kids up?
If so YANBU
Or do they respect your rules and boundaries but just play with the kids and not tell them off, not tell them to eat their vegetables/ quieten down, not put them to bed because you're there to do that, look to you for an answer when they ask for something they're not usually allowed etc.?
If so YABU
Grandchildren aren't toys and women are often told on here to become people pleasers and put wishes of grandparents and other extended family first despite the fact that doing so makes the younger woman's life harder and often makes the children miserable the next day.
This is really hypocritical as we tell women to stand up for themselves and not be people pleasers in every other context.
On the other hand grandparents don't owe their adult children childcare.
So it really depends what's happening in more detail!
Don't feel pushed into accepting something that consistently makes your life harder on a regular basis though!