My mum and dad used to frequently go on our cheapy Sun caravanning trips with us (separate cars), upto 5 days/4 nights at a time, usually twice a year. We always had a great time together- my parents got more 'fun' time with the boys (now 5 and 7) and, on trips out, there were 4 pairs of eyes keeping a watch on things. We'd all muck in together by and large, nothing formal about the arrangements. They'd babysit, usually after 8pm-ish ie after all the hard work had been shared, then they'd watch TV and do their usual evening stuff and we'd go out to a pub (sometimes with dad) or the 'entertainment'. Often that'd be once, maybe twice per holiday, otherwise we'd eat in together or go out all together. For me the great advantage was in the extra pair of hands (mum's!) helping with the boys, and the 'payback' for mum and dad was in quality fun time with the boys, watching them enjoy the beach or a castle or whatever and a trip awayfrom home, with DH and I kind of making the decisions, doing the booking etc.
I must say I did have a good relationship with my folks (mum does pick up 2x a week from school for me, til 5pm). Dad died a year ago and we wondered how mum would feel about the trips, but she's carried on coming along, the main difference being we'd only leave her 'alone' ie babysitting one evening out of a long weekend, say.
In short, it very much depends what your mum's relationship is with her grandkids and you. You say she's brilliant which is a plus! My advice would be not to get too heavy into the contract-y stuff, just play it by ear on a day by day basis. I think it's OK to say 'Is it OK if you keep an eye on 'x' til lunchtime?' so everyone knows where they stand but you'd probably find, like we do that just having someone to take that suddenly whingey child off your hands for a bit will lighten the load, even if you're actually doing stuff together.
Be careful of making assumptions such as, I must say 'aside from breakfast and babysitting each night' which is actually QUITE an assumption! If I were granny I'd find that a bit much, tbh, and as for babysitting every night, does that require her to sleep in the same room or vicinity of the children? As others have posted, you take along a nanny if that's the level of help you require!- but it can be a win/win situation IF you remain sensitive to each other's needs and wants and accommodate accordingly.