If you want to handfeed them, you'll need to keep the parents away from them for an hour or two first so they'll have enough space in the crop to take some food. I find that the best time to handfeed chicks that are still being parent fed is first thing in the morning, before the parents have had time to stuff them full of food. This means that my own breakfast has to wait lol.
Don't expect them to gape for you. Cockatiels don't gape the way that many wild birds do, they have a different style of begging. In the beginning they'll have no idea of what you're trying to do, so they're not going to beg for you to feed them. At first you'll have to sort of force a little bit of food in their mouth to give them the idea. Make sure it's just a small amount, and give them some time to taste it, figure out what's going on, and swallow it. Then give them a little bit more. Eventually they'll understand completely and you can feed them faster. They aren't going to want as much food as a baby that's being completely handfed.
