Ok, I've been reading all your replies - gratefully received thank you
- and am really surprised at how many people agree with me in the childcare element of things. I thought I was being mean and it would come across that I didn't love my DGD if I was to refuse to have her. I also take on board the points that argue childcare enables GPs to build a closer bond with the child, yes.
GiddyZackHunt I think they could afford childcare for one child.
TheCraicdealer I agree, with hindsight, it does look like I've slept walked into this situation and agree with you, and others who have said it, that I need to have a discussion now.
Obsidianblackbirdmdknight I can see why you would think my attitude towards childcare costs is irritating but it's more a case that DS doesn't want to take parental leave (loves his job) and Ddil doesn't want extended maternity leave.
CauliflowerBalti One more would make all the difference because it would be a newborn. I know I have one under 10yrs old but a toddler and newborn would knacker me out and leave me nothing to look after my other two at the end of their school day.
confusedandemployed
at your playgroup conversation - how heartless!
DreamonaStar DS is a very strong minded character and usually gets what he wants.
bakingcupcakes I'm always delighted to see DGD so you are correct in your thinking that they wouldn't know I'm concerned about a second arrival.
JizzystradlinDDIL doesn't want a long maternity leave, she says she would miss her job too much and needs something for her over and above being a parent.
MrsJayy If I said no DS would say he understands. However knowing him the way I do he would make a point of not asking me again as he would deem me unreliable.
MyBigFatGreekYoghurt I highly doubt he sees that as a task in his 60s!
Klaptrap In the beginning it didn't feel like childcare as such but the cumulative effect of having two in my schedule would begin to take it's tool quite quickly I believe. If I did not agree to looking after the second child then they would ask the other GP and I'm sure she would say yes.
Blondeonatreadmill Young parent, young grandparent. Yes, maybe you're right.
Waitingforsomething I said yes because DDIL returned to work 8-10weeks after the birth (IIRC). She and DS really wanted DGD to be cared for by GPs with her being so young as it would make them feel more comfortable. They didn't feel able to trust formal childcare providers at that point.
mummygiles When we're sick or going away they are usually understanding but then I give them lots of notice. I.e. We have confirmed dates for the remainder of this year and they know when we won't be around.
ontheholidays DDIL already works part-time and DS works 6 days a week, a condition of his contract and TBH I don't think he wants to change that because he enjoys being at his job.
schwabischeweihnachtskanne I didn't consult DM about making her a GP, I didn't even plan it myself! 
Thisismypassword We cut the financial umbilical cord after we paid for his wedding and honeymoon.
ProfGrammaticus I've always worked and paid for my childcare in relation to all my children including DS so he hasn't seen a precedent set there. I was a single parent with DS and his childcare costs left me skint every week on my wage but I never expected my parents to do the childcare. 
Thank you everyone for your responses, it has given me a totally different perspective and I can see I'm going to have to get firm with our position and plans going forward in order to manage expectations better with DS and DDIL. 