That sounds like a wonderful project! I've never painted clouds on a ceiling, but I would imagine all the same rules apply. The only difference is that to make the clouds "believable" you'll have to paint them as you see them above you, not in front of you.
For reference, take photos pointing the camera straight up. Also, a lot of Catholic churches have the ceiling painted with amazing scenes of angels and saints that appear to look down from the clouds. You may want to look up church ceiling frescos for inspiration.