I think she's had time to think about it, whether hours or days. Her kids are only young and if she said "Mikaeel's at grandmas cos he's poorly" why would they not believe that.
Because kids aren't stupid, really. It's possible mine are just super-close, but they will NOT go more than 24 hours talking to each other. We've had days were A would go to their grandparents for the night, and we'd swap A for B and B would spend the next night. If the kids didn't see each other for at least an hour during the switch, B wouldn't go.
If mum told them he was at grandma's and then said he was missing, the kids would certainly question and mention it. The lie would have bought her a few hours, but bit her in the *advertiser censored* immediately afterwards.
I do think the simplest explanation is the likeliest here. Rosdeep was at the end of her rope, not coping but afraid to admit it. There was a perfect storm of stressors- Christmas, ill (and sounds quite clingy) kid, court case, the possibility of a Child Services investigation if the neighbour's story about leaving them Sunday night is true- and it was just too much. Instead of walking away, she lashed out and Mikaeel paid the price. Then she covered it up, because she valued her own freedom/denial/for the other kids' sake/whatever more than she valued Mikaeel.
Given the fact that she only lost custody of some of her kids, I would be surprised to find the evidence of long term physical abuse that was apparent in (for example) the Baby P case. Not impossible, of course, but this reads more like abuse that escalated to me.
Also, am I the only one kind of horrified that the foster parents have been named and a quote from a friend given? While I am sure they are devastated, I don't know a foster family (who are admittedly North American) who hasn't had "don't give identifying information to anyone" beaten into their head.
