How much oil should you use when stir frying a single portion?
Use about 2-3tbsp for a 'normal portion', and reduce that by the same factor as you reduce the portion for your daughter, e.g. if your daughter's portion is half the size, use 1tbsp.

Remember you can always add oil, but you can't take it away (not easily anyway), so err on the side of caution and whack a bit more in if it seems necessary.