Well, you could always wait until she had a toilet fail, which, if she times it right, would be about the time it is your turn to be served, and then you could say to the person nearest you, “Could you just hold her for a moment while I get my wallet? I don’t want her running away.” All with a winning smile, I might add...
:o])
Fortunately for them, the online payment went through the second time ;-).
I remember once getting eager assistant from a landlord - who had been refusing to take my calls to that point - when I threatened to go to their office with all the children and their goods and services, including the potty seat.