Grumpy - I am also exhausted even though I have Granpal as co-childminder. It is a very difficult dilemma. DDs both working 3 days a week now yet it would scarcely be worth their while financialy if they had to pay for nursery for 3 full days but their salaries are necessary to pay the mortgage etc. Our schedule is very complicated as 2 DGC are now at school nurseries but at different schools 4 miles apart. Both go in at 9.00 and come out at 11.15. Private nursery, where afternoon session does not start till 1.00pm only takes them up to 5yo. DGS will be 5 in February but will not start school until August, then 6 weeks of half days, then 'full time' = 3.15mp finish, his cousin will be the same but at a different school and his wee sister will start school nursery in at 9.00 out at 11.15.
Sorry, I bet you are exhausted just reading that. There are further complications with in-service days, Gymnastics classes, dentist's visits, sick children, concerts at school, etc. etc. and dilemmas such as dividing our time equitably between DD with one child but no other Grandparents and DD with 2 DC but another set of Grandparents who come 40 miles to cover one day a week. Even if there were plenty spare money there would still be problems with transport between school & nursery and lack of nursery space on a casual basis.
So - it is impossible to 'just say no'. There really is no other alternative.