I think there are 2 issues here?
Looking after your youngest grandson during holidays and the lack of care and consideration shown by your daughter?
You don’t say how old you are and we are all different - my DGSs are 4 and 8 and I never have them together here, although I meet up with my DDs and them at various places.
How old is your DGS - will this continue for many more years? DGS1 attends a Holiday Club at his local Community Centre and thoroughly enjoys it.l Would that be a possibility, say alternate weeks in the long Summer breaks? When DGS1 is too old for the club, his parents will simply stagger holidays from work and I’ll help out, if I’m able. You say ‘youngest’ so does he have siblings at home, who might soon be able to take care for him, or take him out to give you a break?
There’s also no mention of his father or any paternal grandparents?
As for your DD - what happens at handover time? Does she pick up her son - does she have time for a cuppa, or is it all at breakneck speed?
It does sound as if she takes you for granted and seems to have forgotten her manners with not a word of thanks.
As others say, resentment can creep in, so as the School holidays are known well in advance, now might be the time to discuss child care for half-term week, or make a dental/nail/hair/chiropody appointment (or all 4) for that week and then plead ignorance!