to be cross with parents in law for not taking 7mo ds's sleep seriously?

[Lirogiro]

MY mil and fil look after my 7mo ds a couple of afternoons a week. They are lovely and generally look after my son well. However they like to be doing stuff with him so don't prioritize his sleep.
My ds will go to sleepin the car, his pushchair or will be rocked to sleep and go down in his cot, so they could go out with him as long as they gave him an hour or so in one place to have a good nap, but it's always whistling about, 15 mins in the car, 30 mins in the shops etc.
Usually I let it slide because they are doing me a favour but today it's really got to me. They'd agreed to have ds for an hour for me to get a haircut but then asked if they could have him for the whole afternoon. I said that was fine as long as they gave him time to have a good nap as he's getting over a cold. They said they would but when they dropped him off it transpired that they had been all over the place as usual and ds had 10 mins sleep in the car.
I feel really angry about it because I asked them specifically. Aibu? And how can I address it in a nice way that they'll take notice of?

Oh and as for "Think yourself lucky" comments, just because the OP has free childcare if she wants it doesnt mean that she should put up with having her wishes disregarded.

Jealous, much?

have you been clear enough about the consequences of him not napping?

have you actually said "he doesn't seem sleepy but if he doesn't get x amount in the morning/afternoon then he'll be grumpy all night and DH and I won't get any sleep (and poor DH will be too tired for work)" or something to that effect?

if somebody said to me before having children to 'make sure he naps' then i'd let him fall asleep if he wanted to and not really know what to do if he didn't seem sleepy.

Thank you for the range of responses! I think I am particularly annoyed because they agreed to give ds a chance to sleep but didn't. I am not strict about a routine but there is only so long a baby can go without sleep and he defo gets over stimulated and wired if he doesn't sleep and then it affects the rest of his day. I think I'm going to have a chat to them about the importantance of sleep to babies.
Btw he is my fb and is absolutely precious. Aren't all children. I am also well aware that this is a minor thing, but that's what mumsnet is about - the big and the small stuff coexisting together (in the same nest with all the vipers)