OP - while you working part time might work for your family, if you are to raise the sort of money you need, you'll need to remove your DC2 from education one day a week for castings/work. Do you think that you being at home part time would be better for DC2's education that 5 days a week at school?
Seriously, either you aren't going to make the money via modelling, or you are, but DC2's education will have to become parttime.
You say you aren't great at long term thinking, but you need to start - you have created a situation where your DC2 has an inferior education to DC1 - the fact that you justify that by saying DC1 is brighter doesn't change the fact that DC2's education isn't as good. Next, you are going to try to make it fairer, because DC1 thinks it would be good for DC2, do you think your DC2 would do well at that school? Had it not occurred to you sooner that it was highly unfair to give the extra academic help (through being at a better private school) to child who needs extra help academically the least (DC1), rather than the child who needs it the most (DC2)?
You seem to have decided that academic results don't matter for DC2 (I really hope it's not a case that DC1 is a boy and DC2 is a girl). If they aren't as gifted, they need the extra push more, not less. Removing them one day a week for outside work is going to mean they are even less likely to do well accademically.
Really, a mum who works full time would be better for DC2 than having an inferior education experience (either by not having the great private school or by going there but losing a day a week to modelling/castings).
also remember htat castings and model shoots won't be normal school hours, so you will still need to make sure you have paid for childcare for DC1 for the days you are at shoots/castings with DC2.