Because she knew this was a problem in march. She put in the holiday in April but didnt get Christmas.
There was a possibility this could happen in April. She also knew she might have to start working by then weekends too.
Did she formulate a back up plan?
No
Ultimately it won't be her child that pays the price. She will clain unpaid carers leave. either the colleague will cover or the poor person the OP cares for at work will go without care.
It's breathtakingly thoughtless. As a carer, unpaid or working, you have a responsibility to those you care for. You have to have a back up plan.
When my DS' doctor sat me down to tell me DS's diagnosis , (funnily enough barerly verbal ASD although hes actually become hogh needs since), one of the first things he said was you will need to consider whether you are able to work. If you have a high needs child someone, usually a doctor will tell you you need to claim DLA. By the age of three my DS's nursery placement failed. The reality of a proper high needs child is impossibility of childcare and the fact that working is actually a luxury. Especially as a single parent.
First thing I did was sit down and do the sums as to whether I could afford to live on a carers salary and benefits because there wasn't a lot of choice.
Just from what the OP has posted here that if she is better off working at all I would be very much surprised.
Either the OP knows this and working is a choice. Or the child isnt as high needs as the OP suggests. Or she hasnt sat down and worked out what the alternatives are.
If she hasn't she should. Because 4 years is about the easiest time to get childcare for a high needs child, it only gets harder.