Obviously it's too much but given she's already asked her employers and they've said no, comments like 'she'll just have to change her shifts' aren't helpful.
I would do it but I would do it for a very short, defined, period of time. I wonder if she's made a formal request to her work or if it's just been a verbal no? If not, that's the first thing she has to do. She can do it before she goes back to work and they have to consider it and give reasons for refusal.
Next she has to look at their mat policies for coming back to work. Employers sometimes often a phased return which could give you a couple of months breathing space, ie she could phase back three days a week for the first couple of months which could mean she stays with you for half the week for a bit, which will mean everyone gets a bit more sleep! Or she will have accrued holidays during mat leave, people often then add those to the end of their leave but actually in this situation it would be better for her to take two days off per week for the first few weeks and do the above.
BUT THE MOST IMPORTANT THING IS she has to commit to finding a new job if none of the above works or if it's only a temporary fix. She'll know within a couple of weeks if this is going to work out longer term. So you need to be really clear that you aren't offering this over the longer term, it's too much for you, but you'll do 2 - 3 months, preferably part time, until she sorts another solution, whether that's better hours, different childcare or a new job.