Congratulations on your 4-month-old knees. If you're taking plane trips and caring for babies, you ARE doing well. That's a lot of activity for 4 months out and it could be why you continue to have some swelling. But, some people just do. In my case I had swelling for more than a year. It didn't really stop me from anything, but it could make things uncomfortable now and then.
Just be sure you still kind of baby your knees a little for a couple more months. Your body is still doing a lot of adjusting in terms of alignment and soft tissue readjustment. Try some ice for the swelling.