OP I am not going to point out the error of your ways, you aren't the first to presume how admissions go and be wrong and you won't be the last.
What year is your older DC? Again the ICS could apply to them if you try and move them to a different school. Arrange to visit the offered school, get a feel for it and consider the possibility of moving your eldest DC there too. Speak to the catchment school too about both children.
I am not sure how realistic getting a place is from the wait list, it depends on how the places move in your area, so you need to have a plan for the youngest not going there. What is before/after school provision like? Either in school or with a cm.
A word of caution though if you are planning to try and move your eldest a space generally has to be accepted straight away, they couldn't 'hold' a place until they both had one. I have this issue with my youngest as I am trying to move school for her and whilst the school has a place now, I don't want to move a yr2 child at this point, plus I still have a yr6 at her school (we've moved up the road)
Good luck, take stock and work out a plan
It will pan out one way or another.
I am however one of those people that did only put one school for my youngest, but I had a child in the school, plus we could see the school field from our house, so the chances of 56 children with either looked after status (etc) or lived closer with siblings was pretty slim (especially as I knew that really wasn't the case) I would never advise others to do it.