I've flown my son regularly since he was 10 months old, strapping him in using his car seat. Just like a road trip, the white noise and vibration helps coax him to sleep. To protect his hearing I bought him a Soft Comm headset built for kids with in-built ANR. At the time it was less than $US200.
My most valuable suggestion is not to sit the kids in the seat directly behind the pilot, if possible. He woke up once when I was on short final in a FW plank and he decided to kick my seat with both feet until he gained Dad's attention. His kicking continued as I was trying to land in a howling crosswind
Now he is almost 4 he loves every trip I take him on. I even give him a sectional chart so he can 'help' navigate. I couldn't recommend enough the experience of taking your little ones flying, whether that be RW or FW. It will give them memories to last a life time.