One huge positive that came out of today was that Jack-Jack's mum asked the sessional worker to write a note back to us in the notebook that we always send with him to contact. We've been writing what sort of a night he's had, what he had for breakfast, anything "wrong" with him (rashes, teething, cough etc.) and always that we hope they have a good time together at the end. Apparently his mum was really pleased with the notebook, and she asked some specific questions about Jack-Jack in her note. Now that I know how she feels, I can write a longer, chattier note next time, answering her questions and giving a bit more detail about Jack-Jack's day.
I feel a huge amount of relief that this first contact has now happened, and that there doesn't seem to be any animosity from mum towards us.
We have another milestone to report - this evening Jack-Jack pulled himself up onto his knees during his bathtime in the kitchen sink! Thankfully the bath seat we ordered for him arrived this morning, so we'll spend the next couple of days getting him used to it, and then he can transition up to the big bath upstairs. Honestly I wouldn't be surprised if this child stood up and took his first steps before he could properly crawl!