Hello!!!
I've travelled by air lots of times for a couple of hours and there is no problem.
It is true that you feel different on the out flight when than on the return flight. You feel different in the same way that you do when you are not flying. I mean, when your Hb is lower, you feel like you have more difficulty in breathing but that has nothing to do with flights.
In fact, in one of the flights, something happened with pressure on the plane and some of the guests needed some oxygen. I was there, my Hb was around 9 and I didn't need it.
So, don't worry. I'm sure your child is not going to notice any difference.
Kisses,
Laura.