I’ve never flown with a baby that young, but think of what she likes to do with her hands and plan accordingly. When my son was 13 months old, we flew halfway across the country and I worried that he’d drive everyone crazy by being unable to sit still on the plane. What kept him busy for most of the flight? Cheerios and a small box. He’d stuff some Cheerios in, fish them out, eat a couple, stuff them back in, fish them out, over and over. As long as he had Cheerios, he was happy.
Try to pack some small toys that your baby has never seen before. The novelty will keep her attention longer. Also, you can get some of that “mess free” art stuff, like Crayola’s Color Wonder (I think that’s what it’s called). The markers only work on certain paper and don’t make a mess. Even an 8 month old can scribble with a marker, and your three year old will keep busy with that as well. If you have room in your carryon for some small picture books, that’ll be great. I recently flew with my 4 year old, and had him bring a backpack of toys, Maybe your son could wear a backpack, and also carry a few fun things for his baby sister. That way you aren’t stuck hauling around a huge bag and two small children.
If you have a layover, find a place where your kids can move around a lot between flights. I took my son outside the airport during a layover and had him run in circles, jump around, and basically use up some energy.The downside was we had to go through security again. If you can find a spot for the baby to crawl or scoot around (if she’s doing that), she might be happier to sit when you’re back on the plane. I second the suggestion about bottles, and I really believe that children on planes can never have too many snacks. Not sugary, bad things, but crackers or cereal.
Just as an FYI, in case you haven’t flown for a while, the airline may not provide snacks unless you pay for them. We flew United, and the snacks were $3 each. You could get a snack box for several dollars more. I was so glad I’d packed snacks for my son instead of planning on having them provided on the plane.