I know from VERY bitter experience that most young children cannot hit the target with an airsickness bag. Take a small sand pail onto the plane with you, and keep it handy at all times. Beware of just after landing; that is the most common time for a child to throw up, especially if his/her ears have been a problem on the flight. Just sit and wait until everyone else gets off the plane before you start to move.
Withhold dairy starting at least 2 hours before the flight, and try to get decongestants that do not have added dyes. With a sinus infection, the most effective decongestant is one that contains guiafenesin (Robitussin and generics of it), this will usually be combined with sudafed.
Avoid greasy foods and fruits that are slippery, like peaches. I'm sorry to get somewhat graphic here, but also stay as far away from cheese-flavored crackers or sweet cookies as you can get. Once the medicine kicks in, the child's stomach will get a lot of sinus drainage in it, and that is a high-risk trigger for airsickness. Feeding plain starches like pretzels, saltines or cheerios should help to absorb the gunk.
If the ears start hurting anyway, ring for the FA and ask for cups for ears; they will know what you are talking about. (You will get two foam coffee cups with napkins stuffed into them, that have had boiling water absorbed. Tilt the cups first to check that no excess water will dribble, then put them over the child's ears like muffs. The steam will relieve the pressure inside the ears.)
However, just in case the worst happens, put lots of wipes, a towel, and a change of clothes for yourself and the child in a large ziploc in your carry-on. (The towel is to put on the seat if it gets hit.)