Are you sharing accommodation? If not just tell her via text when you are out and about that you have gone for a mooch with your dcs.
'Just mooching around, not sure where we will end up, see you later'
Then when later comes and she wants to meet up 'Don't wait for us, will find you later if we don't have an early night'.
Hopefully when your other friend arrives other friend will either stay with demanding friend and you can meet up as and when it suits you. Or other friend will realise what has been happening and you can join forces.
Have been away with my friend, my dd and my now dh. It was horrendous. Friend just wanted to sunbathe. Absolutely fine. We told her we go mooching around a lot, she was welcome to come with us or stay and sunbathe.
She refused to sunbathe alone so came with us complaining constantly. And about everything. I snapped at her on day 4, told her we would be walking a lot, wouldn't be stopping every 5 mins for a fag break or drinks and she would be better off staying by the pool and making full use of the AI as we were not buying her drinks and lunch and tea again as we hadn't budgeted for 4.
Money wasn't an issue for her, she had much more than us at the time. We had booked AI as it was cheap but fully expected to eat and drink out of the resort so had budgeted accordingly. But not for an extra person who was too tight to buy herself a drink as she had already paid for AI.