That business about warning people that your kid is prone to airsickness does NOT always work, especially with dedicated aisle-seat people. DS had severe reflux until he was 4, and was GUARANTEED to be sick on himself at least once on every flight, and sometimes several times. Airsickness bags were insufficient; we had to carry a large sand pail wherever we went. He gave absolutely no warning, though, and the bucket was useless half the time. I always sat down and spread a towel over myself and another over his carseat when flying with him when he was little.
On both of the only two occasions that we flew with DS as a lap-child (family emergency, only two seats left on the only avail. flights), he threw up all over the passengers in our row who refused to trade with DH. I told them that I did not at all mind sitting in the middle, but that there was no way I'd be able to redirect his aim from both sides; one or the other of them was going to get hit. Those folks ended up being very upset with me, especially on the very turbulent flight where the FA would not let either of us out of our seats to clean up afterward. (DH had window seats on both occasions, but toward the rear of the plane. I tried to avoid the window in those days, b/c of having to step over people en route to the head to clean up. I tended to drip on them no matter how hard I tried not to.)
BTW, someone earlier posted that there is an FAA rule that says kids under 5 must be seated next to a parent. Not true, I'm afraid. That rule only applies to children who are strapped into carseats. Some airlines have their own rules re: kids under 5, but the rule usually is that they must be seated in the same cabin as the accompanying adult. FA's will usually try to bribe people to get one parent next to a child that age, but they can't force it unless a carseat is involved.