There's very little official guidance on precisely how much food children require, so parents need to use their own judgement. As a nanny however, you have the bonus of having the parents' experience and insight to draw on, so ask them about the portion sizes they give their children.
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In general, it's best to give children small servings, and let them ask for more if they're still hungry. Most young children can regulate their own appetite so encourage them to eat but do not force them or expect them to eat if they are not hungry.  For toddlers, who might be less able to let you know if they're still hungry, this guide offers some handy portion size suggestions.