Yes I agree with magicmystro and Soren on this point: Grab their attention.
Mind you, every opener should be of this ilk, but really go for it. Save your best for the toughest.
Flash paper. Or fire of some description.
Personally I use a visual transpo or a colour change.
There are other methods - I change to gambling patter if I encounter recalcitrance in adults. It seems to get the blokes on my side. I don't use anything 'spiritual'... though I have seen it work with great effect on disinterested women. I have a friend who has
one tarot type patter. Just the one, that he saves for nonchalant females (in fact, and don't tell anyone this, but his most of his sex life was also based on this one patter).
Secondly, as Soren also says, some adults see the magic as just kids' entertainment. I have been openly asked (nay, told!) to "keep the kids busy" while they enjoyed thier lunch. Pah!
Mind you, with the manager watching - and paying your wages - you don't have much choice. But keep it close to the adults - if you do something eye catching they
will end up watching. Even if they are doing it in a way where they are just pretending to watch along with the kids. You know - like taking your nephew to see Bambi just so you have an excuse to go to the cinema.
If this happens then let them do it. After all, they are watching and interrested but don't want to admit it. Result!
