How long is your flight? You don't want room for food and water if you are carrying your baby on with you. If you need to feed them or give them a drink, you'll want to do it in the airport or if absolutely necessary the airplane bathroom (with a wee wee pad on the floor to keep your baby clean, of course). Remember that you cannot carry any liquids through security, so once through you'll want to get a bottle of water to bring on with you if your baby gets thirsty. You're really not supposed to take them out of the carrier or take the carrier out from under the seat at all, but I can't see leaving them under the seat if you get up to use the bathroom - so just use that as an excuse!!!
To carry-on your baby, you just need to make sure your carrier is fully enclosed, has adequate ventilation and will fit under the seat in front of you. Each airline varies with requirements, but most to of the time, you want the height of carrier to be between 9 & 11". You also want the carrier to be able to stand on it's own (meaning the sides don't collapse if you put it down).
__________________  Sissy & Angel |