Children's nutritional needs vary according to age, gender and the amount of exercise they get. And it's a misconception that boys need to eat more than girls (until puberty, at least).
For children under the age of 2, pediatricians recommend gradually introducing a wider range of foods into their diet. As far as quantity goes, your child will let you know when he or she is full: most of the time when they turn their heads away or start dribbling, they've had enoughl!
From 2-3 years onwards, children's needs vary according to the individual. Trust them to let you know how much they need! Don't force them to finish what's on their plate, and give them seconds if they ask (but make sure they get a healthy balance: for example, give them a piece of fruit rather than another piece of cake). Stick to regular mealtimes and take time to sit down and eat at the table, quietly, so that your kids can tell when their bodies are sending out 'hunger' signals or 'full' signals.
You should never put a child on a diet or refuse a child food under the pretext that he or she is too fat: otherwise, they will simply overeat as soon as they get the chance in order to compensate.