This is an eternal question and enforcement seems to be spotty and much debated here. The rule is that you should be able to ride in the same "ride vehicle" - which is a single car with no coupling between - with the child under 7, however on many rides the "same row" is enforced. 7DMT seems to be one where it is by row. Barnstormer is also a difficult one because there is only one row of two per vehicle. On other rides it seems to vary. Space Mountain is single file, but you should be fine since all three of you can be in one vehicle.
None of this has anything to do with height and be prepared for CMs to ask your kids directly how old they are. If you're comfortable with it, you may be able to ask a CM to group you with another odd-numbered party who is willing to sit in a row with one of the kids, or just chat with people in line and see if you can work something out. Most people are really nice about it if you ask. I've seen it many times. Anyway, best of luck and have fun!