This is a common way of asking a child how he or she is.
Using this phrase also reveals that you anticipate the answer to be between 1 and 10, since the interrogative word used is 几 (jǐ).
If you anticipate the age to be bigger than 10 then you should use the word 多少 (duōshǎo) instead. Or if you just want to learn one phrase which will not - I hope - offend anyone, then try this one
How old are you? (universal).
