She is proposing to ask another Mum to look after your 3 children so she can go to a concert. On top of their own children. FFS
Or is she proposing to farm all three of them out to three separate parents, presumably for you to pick up on your way home.
Err - that would be a big fat NO from me. If she has screwed up her entitlement for this year by badly managing it then frankly it's tough.
We'd all like extra paid leave, nature abhores a vaccuum and all that.
There are options - she accepts a lower annual salary in return for additional leave [assuming you can manage the gaps] next year. You vet a reliable babysitter who can cover the odd day here and there [which I would recommend from the sound of her anyway] to cover sickness/random days off.
Final option - Net cost of the day off comes out of her salary or she pays to you. An emergency nanny from in London London is circa 20 phr to cover taxes, transport etc. You'll soon see how keen she is to see Kanye West or whoever.
New contract for next year I think. Min notice periods for requesting days off and an absence/sickness payment policy if you don't have one in place already.
I would remind her that she's already had an "exceptional" holiday for this year, one that did totally inconvenience you.
All that said, I'd also remind yourselves that you do require a lot of flexibility from her if one of us travels then overtime required. Perhaps you need to give her the option of time in lieu instead of payment at the time? Overtime banked could be used against a babysitter cost.