i assume that children that have put the comprehensive school as their first choice will get the first pick on places.
No this is not at all how it works.
Children will get the comp places by living close enough or having a sibling and if it hasn't been possible to offer them any of the schools they placed higher on their form.
So, in your case, if you live close enough to definitely get a place at the comp, you can place it last on your list and know you have a safe option just in case none of the grammar options work out.
You cannot get pushed out by people who qualify less (eg live further away from the comp than you do) but who listed it first.
You should list the grammar schools above the comp in the order you like each school. It doesn't matter if your chances at some of them are lower than at others. If they cannot give you your first choice, it is crossed off and your second choice becomes your new first choice and so on until you have a school.