Ok, well it seems pretty bloody obvious the CM was lying, in which case:
She will have falsified her OFSTED records for that day.
The safeguarding issues are clear and present (physically in event of something going horrendously wrong; emotionally and mentally - both already perpetuated
upon OP's DS).
She has committed theft with intent to defraud - you can have no clue right now as to the extent of both as the chances of this being the very first time seem minimal. Even if she only' did this once a month then that's a clear grand stolen (& de facto not also declared to HMRC).
You do have a Duty of Care to report to: SS, OFSTED, Police. Whilst I sympathise (as clearly you never asked to be in this position), you genuinely have no choice.
And if Bill is such a whizz on Fraud, then he/she would know this is Fraud, albeit one that would be investigated on a local level; likewise that what makes this fraud so very much more serious are the safeguarding issues wrapped up in it. There is no way the Police will not investigate (& they can/will speak to other parents so I'd leave that well alone) and will prosecute if can obtain sufficient evidence for CPS - something that looks like a slam dunk 'just' from this incident.
You CAN'T not report - there are other children at risk here (and no, you cannot assume the lying is restrained to simply obtaining money fraudulently; she has already demonstrated can and will fuck with the heads of her charges, so for all you know of there may potentially be worse going on behind closed doors but she has also manipulated children vis that). What this incident does is throw up her every self-reported entry in OFTSTED daily register as now questionable and you cannot ignore that nor it's implications.
I'm so sorry OP 