Would love an update on how you handled this.
I think its best to not rock the boat and to do things their way as it is their child.

I would sit her down and express your concerns. If your grandbaby looks like she’s not growing or malnourished I would have big concerns.

I think like others have said, a talk with both parents is in order.

Sit down and talk with them and explain your concerns about your Grandbaby wanting more to drink when she’s with you overnight. Make sure they understand it’s not criticism but you worry that she doesn’t sleep properly because she can’t settle. Maybe come up with a few solutions between you and work out what will produce the best outcome for all involved.

I breastfeed both of my girls and they were fed on demand, usually every 3 to 4 hours. up to the age of 12 months. If my mother looked after them, then I supplied formula. I also found the more I breastfeed the more milk I produced

I think a discussion with both parents is need so you’re all on the same page.

Maybe ask for some extra milk store to trial if she sleeps longer etc. It is very hard to know how much they are getting when breastfeeding and you will always express less then the baby can get. Does she exclusively bottle feed when she is with her mum?

I would be feeding what’s needed whilst she’s with you, to make a happier time while she visits you. 10 hours seems to be a rather long time without milk for a 5 month old

I think it’s important to voice the concern and just ask for more milk. You are doing them a huge favour babysitting so that they can have a life, which surely is appreciated. All well and good if they want to feed less while in their care but they should be supplying you more than what you need, just in case.

I think its your daughter decision and she knows what is good for her baby.

With breastfeeding, the baby stops when she/he has enough. One can’t tell what volume has been suckled. The baby may have suckled more than what’s been offered in the bottle. Maybe a trip to the Child Health Nurse may help.

I think that in your position I would say that I need a little more milk to feed your baby as she is crying so much that it is hard for us to hear this crying baby. Maybe your son is the way to get this through, but no child should be crying because of lack of nutrition. Perhaps you could ask how they cope with a baby who is crying for more and follow their maxim – take the baby for a walk in the pram, give the baby a late night bath or play longer with the baby so the baby is tired out and will sleep longer – they must be doing something different so that the milk regime they have implemented will work.
Put the onus back on them and say you are doing everything you are aware of – is there something new they are are doing that you aren’t aware of.

Your daughter in law seems a bit selfish.
Maybe you can speak to your son about it instead and see what he thinks?

I think you really need to communicate this to grand baby’s parents as there is no point sneaking in formula. Sounds like a really tough predicament but you must respect the parents wishes, perhaps you can really stress just how hungry grand baby appears.