I trained Judo in my high school's Judo club. Our high school was physically joined right next to a middle school. And so the middle schoolers would attend our Judo class also. While I was 5 foot 10 inches tall and about 140 pounds, the middle schoolers were probably barely even 5 feet tall and about 90-100 pounds. Huge difference.
Our instructor paired us up with kids our own ages and sizes usually, but occasionally I would have no choice but to partner with the younger kids.
My advice:
First, try to partner with the instructor whenever you can. If your instructor doesn't do that in class, then you have no choice but to find the strongest, tallest, heaviest children if you can.
When you do partner with children, keep in mind that they are children. That means do not lift them very far off the ground, despite being fully able to do that. Don't throw them long distances. Don't go really fast. Don't fall hard on top of them into your ground holds. If you do those things, they will be afraid of you and won't want to partner with you. So be gentle and work on NOT using your muscle power but instead using graceful technique, fluidity, and leverage.
Above all else, relax! Keep good posture, etc., but work on keeping yourself from tightening your muscles and powering through stuff. Especially when you're on the ground rolling. When you have such a huge advantage over them physically, you need to just let go of your athleticism and instead learn to flow, gently. Give to any resistance you feel, and instead look for the smart way around obstacles. This will also help you when you do eventually encounter an opponent your own size, by the way.
When you are receiving their throw, be ready to protect your face, head and neck, because their throws might end up going short due to the fact that they're not used to the extra weight, and you can fall in an uncontrolled manner. You should practice the one-armed cartwheel and dive-shoulder-roll breakfalls. Those will help you recover from a bad throw done to you.
You might need to squat down a bit by lowering yourself at your knees in order for them to actually be able to throw you. But try not to do that habitually. Let them try to throw you normally. Only offer to lower yourself if they seem frustrated. Of course, do whatever the instructor tells you to do.
Definitely do not jump into their throws, trying to be helpful or trying to make them look better. Jumping into a throw does them no good, and it's frowned upon by them and your instructor. They are learning a lot when they get to partner with someone much larger than they are. Don't take that learning experience away from them.
As for your ACL tear, make sure your knees are ready for practice ahead of time. That means doing about 10 minutes of "functional stretching" and warm-ups before class. You'll practice squats in place, lunges while walking across the room (do whatever height you feel comfortable with), running in place or around the gym, jumping in place, jumping side-to-side on one leg and jumping jacks, seiza posture to standing posture and back, shoulder roll then stand, and anything else you might want to throw in there. You're just trying to put your knees through the motions and stresses that you might encounter in a typical Judo session. Obviously, if anything causes you pain, stop right away.
Hope that helps.
