It looks like your wife has two problems here.
1. Being asked to attend a meeting on here day off without adequate notice to arrange childcare.
Can she not explain her position to her manager/meeting organiser, while making it clear she is being asked to come in on her day off and if she can dial in to the meeting via phone/Skype/other? If you really cannot arrange childcare then ask if she can bring the baby in. This will show she is serious about the childcare issue and not taking the mickey. It also shows willingness to be flexible.
2. This is a longer term one. It sounds like either her performance has suffered – in which case she should ask for a quantifiable measurement she can use to gauge it – or they are trying to manage her out of a job. This is tricky, and I would seek legal advice regarding my options.
Personally, I would not have agreed to a probationary period. I would try and find out the legality of it. Maternity leave is standard practice. Being put on probation once you’re back is one I have never heard of.
For your sake, I hope these things get sorted out. Having to worry about finances is not a fun problem to have. Be the clear head in this situation and don’t do anything rash. Good luck.