when to stop

Surely loads of people work Monday to Friday, but don't stay in bed all day at weekends. Shouldn't they be having a weekend life?

Very odd.

I agree with THEM BOTH taking responsibility , maybe they should take turns to have a sleep in , that is what my lot do

Right, I think that is a bit clearer.
I do think maybe a note in the kitchen to explain you had taken your DGD outside, I could imagine panicking if I got up/woke up and a child wasn't where I expected her to be.
However, and this is a big HOWEVER, a child of 4 should not be unsupervised while her mother is asleep as a matter of course in the daytime, so I think that situation needs addressing. If there were any sort of accident, for instance, your DIL could be seen as guilty of negligence. I apologise for sounding so judgemental, but parents should not BOTH be staying in bed over the w/e when there is a young child in the house.
Many of us were/are working mums so we have been there, race doesn't come into it and if your DIL is so used to having cheap childcare that she doesn't realise that, she has to put her child first then she needs a tactful reminder, not an accident or other emergency teaching her the "hard"way. You say you' have her after school but are having second thoughts about the school holidays. Again, they need to take responsibility for their childcare, if they are both working and earning that gives without saying.
If you do live close by, you could offer to have DGD on a Saturday MORNING to give them a break, but it sounds as if they need to wake up to the responsibilities of a young family.

Thanks all,
It was my DL who said she was confused. My GD will be five on the 26, so just over a weeks time.
I think she wants my GD to have a quiet w/e as now she attends school.
We used to live together when my late husband died, but now I have my own property, so I come over when my GD returns from school on the mini bus. I also have her over the school holidays, but I am now thinking whether to carry on doing this now. I just popped in yesterday on my way back from town, which my late husband and I sometimes used to do, to take her out in the car, while they were still in bed.
My GD was born in Dar Es Salaam, where they had a nanny for her until she came here. Over there, as it is cheap to get help and the well educated ladies are all for working not looking after kids, so, at first, my DL found it rather difficult without being employed in the UK. Also, over there the windows are all wired in, so it maybe that she didn't go out much as a child.

As you say, maybe I should have asked first. It was just one of our first reasonable warm days, so I was surprised to see her still in her pyjamas.

Basically agree with the above posters. I take it you all live together, so you're right on top of the situation, which must make it harder for you.

"She really enjoyed riding her bike over the stones, but it seems her mum had other ideas, as she came out saying that she was very confused. "

I'm also going to echo the question about who DIL said was "confused" - GD or DIL?

If DIL thought GD was confused by being taken outside on her bike, is it cuz that's not usually part of her schedule? Does DIL often take GD out later in the day (after DIL gets up)? If so, then I can see where DIL may have found this disruptive, even though I don't think you really did any harm.

If it was DIL who was confused cuz she didn't expect to find GD outside, well, I know that must have been scary for her. But as ninathena says, "point proved" - lying in bed for hours, leaving a child unattended to is a bad idea.

DIL may not see it that way, of course, which could lead to some tension between you if you were to repeat it. As Alea suggests, better to ask first, IMO. But that's hard to do, I realize, if DIL is still in bed. Perhaps you can work out something with her where you leave a note if you take GD out or agree on a time on Saturdays when you can take GD out on her bike (and how far she can ride, etc.) - or something like that. (Be sure that you don't sound critical of DIL.) If she says no, however, I'm afraid you'll have to accept it.

How old is your grandchild?
I would assume a person from any country would want to be a good parent. Lying in bed all day and leaving a child to amuse herself isn't very responsible in my opinion. I agree with all Alea says.
I would have taken the child outside too. If DiL confusion was that the child wasn't where she expected her to be then I'm afraid I think that's point proved.

Most of who worked when the children were small were tired by Saturday -but lie in bed? Chance would be a fine thing! All day? A lie in even until 8 was out of the question.
No you are not being unreasonable, but perhaps an offer to takes your DGD on a Saturday morning would be a tactful approach, if tact is needed. Not sure who was confused - your DIL? Why?
(Are you posting from Tanzania, or this country? Just thinking, different country, different habits)