Yeah, I only mean something thats enough to get past the mum, maybe with a message on it in small print that the cashier might see like "I've stolen this thinking its real from the little old lady I care for at 1 unknown name street, please call the police on me"
Funny money would be obvious to spot by feel alone, and the wording would have to be big enough for the cashier to see, which of course would be big enough for the thief to see. Realistically, It probably wouldn't even be taken in the first place.
At this stage we don't actually know where the money is going, and how.
The camera option if the OP can get it up and running will show whether money is being given willingly away, under pressure, or downright stolen.
To the OP - try not to tell your mum what the real purpose of the clock is - as she may be doing things with the money that you didn't expect. Just tell her you've got her a new clock, so that anyone visiting can keep an eye on the time, as they're all very busy people. She might also accidentally mention it to the carers - 'Oh my son, he does fuss, he's gone and put a camera over there to watch me - what do you think about that?'.
I would avoid putting peoples names all over facebook etc if someone is identified. Let the law deal with it. Report it, supply the evidence, let an arrest be made. Don't open yourself up to any personal vendettas.
I was in the security industry for a number of years - and I've seen it before - it's just not worth it.
One final tip - if you do find that someone is taking money, save the evidence, and then let the camera keep rolling, and as much as you don't want to lose money, let it happen again. One incident on camera can be argued 'Oh I only did it once, it was an emergency, I needed to pay my gas bill or my kids would starve' - if you have two incidents on camera on different days it's much easier to persuade a court that it's a repetitive pattern of activity, but that's up to you, You might decide once is enough just to get rid of them.
For someone in a position of trust, such as a carer, theft is taken very seriously.