Sounds like a pretty good deal to me. France is a pretty good place to be a working mother - subsidised childcare and plenty of activities for kids on Wednesdays and school holidays, as well as plenty of time off (in my last job I had 43 days of annual leave, including RTT).
Most firms are pretty flexible about staying home with sick kids etc or working from home occasionally, but it really depends on the company culture. IME American-owned firms can be slightly more inflexible than French-owned ones, but it really is specific to each company, and your individual boss.
I've always worked around my kids' hours, but often took work home for later, to make up for not staying late in the office. As long as you are good at your job, most bosses are pretty understanding, especially the current generation.
I would recommend you get your childcare sorted asap before moving, as certainly in the Paris area, there can be long waiting lists for certain nurseries etc. Most kids go to a "centre de loisirs" or sports clubs etc on Wednesdays. Some go to a childminder, but the activity schemes are much more fun
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In some cities (including mine), the school hours are changing from this September to include Wednesday mornings! My DD3's school has an arrangement with a Centre de Loisirs to collect and feed the children whose parents work, but my DD wants to take figure-skating classes instead! (DD1 will probably be taking her, otherwise I would have hired a babysitter, but she's 9, so a bit old for a childminder)
After-school care depends on the school. In our last town, it was available until 7pm every day, run as a non-profit, paid by the half-hour. In my current city there is only a homework club until 5.30, which is free, but my kids walk home afterwards and wait for me there until around 6.30 (DD1 is 14 and usually gets home first).
No idea how much your international school would cost, but the state schools here are mostly great and the healthcare system is fantastic. The state-run international schools are free, but not available everywhere and there is an entry exam in the spring for a place the following September...
In your shoes, I would seriously be considering the offer. I won't be online much over the coming week, but you're welcome to PM me if I can be of any further help...