Your oldest won't count at all, ever, because they are over age 6.

Your youngest won't count during the school year, as long as you have no more than one child in care under age 2. If you're just doing B/A school, then obviously that is not a concern, so during the school year, yes, you could have 5 dckids plus your own.

Come next summer, however, you'd have to drop a dckid because your youngest will still be under age 6, and therefore will take up a spot during the summer break.

No, your own kids under 6 in full-time school don't count during Christmas, March Break, etc, just summer break - as long as you have LESS THAN 2 under 2 (so one or none).