~~Fun but tough! Keep them busy, busy, busy, almost always moving. Some good drills are:
*Making pairs, putting their palms almost touching at face height but out to the front/side and side shuffling as fast as they can down the mat and back: good for staying conected, learning to move feet along ground instead of big, klunky steps, lowering center.
*If they roll/dive yet use groups where one person lays flat and after the next kid roll/dives over them that kid lays flat (as much room as you have for flat kids and rollers), like a snake eating its tail. Good ukemi practice.
*For an easier kokyu tanden ho use a judo randori grab instead of wrists as these can be very frustrating for children. With a more direct connection on the body the kid can feel disbalancing more easily and less chance for their partner to give incorrect ukemi.
*Use foam lengths as bokken 'cause kids love playing with swords. They can learn some basics and then have boffing time where they will actually be picking up skills like moving out of the way, focusing, not being intimidated or calming themselves, etc.
*Dodge-ball is great for perception/evasive movement practice. Have the kids try to dodge using only irimi or tenkan. This is HEALTHY compitition. Gives them a focus, makes them laugh.
*Have them roll/dive to pick items off the mat, or rolling items
*Even though it's Aikido you might teach fundamentals of striking and kicking, or give them balancing drills, or anything to do if you have to work them in groups so that the ones on line waiting are still training something and not just farting around--which is an automatic kid failsafe if they're unfocused for a quarter second.
*It's okay to have one pair pracicing the full technique being taught while another is only capable of handling part of it. They'll both learn something and not be frustrated.
*Have quiet time (meditation) at the close of every session; even the little ones can sit for a minute if given soft voiced guidence or an image to hold. Have question and answer time as children assimilate by being able to express themselves back into the world (as do we all).
You have another challenge because, developmentally, there is a huge range between 7 and 14! Some practices work fine as pairs or groups but know when to break the kids into partners roughly the same size/age. There'll be older kids who have the mental/physical capacity to seriously train and they shouldn't be held back because there are a bunch of wee ones about.
Hope some of this helps, from a one-time teacher of children in MA and a mother