You can of course say no, but have to accept that this will have an impact on your relationship with her, and that she may well choose to place reciprocal hard limits on any support you might hope for from her as you become older.
Possibly you don't expect any - maybe your relationship with her does not involve mutual help and support, and if so that's fine.
If I am lucky enough to become a grandmother, I have always said I am happy to give as much support and help with childcare as I can/my daughter needs. Frankly I would love to!
Equally, I hope that as I age my daughter will, within reason, be happy to do things to support me.
My elderly father recently passed. He had severe dementia for the last 10 years of his life, and went from being a fiercely independent, proud man, to someone who needed support from his family for the basics of survival. After my mother, I was his main carer.
He didn't plan to need this help, but he needed it, I gave it. I sincerely hope never to burden my daughter with even a fraction of the caring responsibilities I picked up - but none of us have a crystal ball.
You see what I am saying.