I used to do one day a week with Gdd, now 22 months, and still do a lot of helping out since dd also has Gds, 7 months, who is not a good sleeper. I take over sometimes so dd can get some sleep! Yes, I do find it tiring, but am glad I am able to help, and do have a lovely relationship with Gdd.
Having said that, I don't think I can cope with two when dd goes back to work, not on my own. So we will very likely be helping with the extortionate childcare costs, instead.
I do sometimes wish dd had had her babies 10 years earlier, when I would have had late 50s energy levels instead of late 60s, but then the time wasn't right for her then, plus I was still working and had a mother with dementia needing a lot of time.
So it's all been for the best.