I predict this will become boring very soon. I agree it is hard to exercise with kids. Here are some suggestions; dance with your kids, put the music up loud and let everyone go nuts. If you have a baby, put them in a backpack or frontpack and go for a brisk walk with the other kids in a pram. Lift your kids up and play with them. Rough and tumble on the bed. They will love it and you will get exercise. Clean the house vigorously. If you concentrate on exercising in your everyday life you will get fitter. Put the washing on a line but bend down to the clothes every time. Make the beds with vigour, dig the garden, throw a ball to your kids, kick a ball around. Take care.
Yes, you are damaging your knees, slowly.
Walking, Running and everything in between is considered high impact exercise, and will gradually damage your knees. The damage builds up so the older you get the more noticeable and faster damage will be.
Your genetics, weight, previous impacts, are just a few of the many factors that can determine when in your life the wearing on your joints will start to cause you pain, in some people it is earlier than others.
Luckily there are many types of compact exercise machines which specialize in non-impact and low-impact exercise, like exercise bikes, cross trainers, gliders, and even small floor standing pedal devices where you sit in your own chair and pedal with your feet like on a bike, all far better that walking, and all adjustable so you can find a resistance level on the machine so you can do exercise that improves fitness rather than overworking you. These should be better for you, and your joints.
The way you walk the way everybody walks, so I can't understand why you feel pain in your knees, maybe you must visit a doctor to see what's happening, walking for a long time only cause fatigue and maybe som breathless if you're not used to do it, but if you keep it doing you'll feel better, also you can try to do more exercises not only walk to get fit. Good luck
My gym has childcare on site. Check out the gyms in your area, perhaps they offer the same service. This will allow you to work out even more stuff. Good luck