Just say No,
She and her husband need to sort their child care out and not expect to inflict their child on someone else leisure time.
It totally changes the tone having children along,
I love doing things with children, which is good as I spend the majority of my life in the company of children,
But when on those very rare occasions, something is arranged that does not include children, I admit I do resent it when someone puts their need to join in with child/children in tow over everyone else's need to have an adult break.
A couple of years ago, a bunch of us booked the youth hostel at Tintagel for an extra long weekend, for a very rare catch up on old times, take long walks, generally kick back relax, one of our dear friends, kept asking if she could bring her toddler, and young children, her husband wanted to take part in an activity, we kept saying no,
you only have to look at the Tintagel youth hostel to realise why it would be an extremely bad idea, ten feet from the front door there is a hundred foot straight drop into the sea, we all explained why it would be a very bad idea,
She turned up anyway children in tow, she had a terrible time, everyone else trying to accommodate her had a terrible time, children had a terrible time as everything was not geared to them, because it is not a suitable place for small children.
Everyone who has children has to suck it up at some point, that what you do, you don't decide you are entitled to inflict your children on other people adult leisure time.
Just tell her she will have much more fun and relax more without a child in tow, and to find some child care.