僕はアルクの月刊『子ども英語』雑誌でCool & Genki Englishの記事を書いています。 英語教室で使う表現を、ネイティブスピーカーの言い方でもっとかっこよくします。
How many members in your family?
In English there is a sort of semi-informal list of questions that people ask children. The first one is of course "What's your name?", the second one is "How old are you?" (because unlike Japan we can't ask kids what grade they are in as during July and August they have no grade!), and the next question is usually to ask about their family.
Of course you're not really a "member" of a family, it's not a club and you didn't get a family membership card when you joined. So asking "How many members in your family?" sounds very strange. "Members" is usually used for things like school clubs.
In Japan we usually ask “家族は何人ですか？”, but in English we usually ask in a different way. It's much more common to ask "兄弟は何人ですか？". In English you could ask "How many brothers and sisters do you have?" but for children who don't have any brothers and sisters that can be a tricky question to answer. So we usually use "Do you have any brothers or sisters?", which anybody can answer "Yes, I have 1 brother", "Yes I have 4 brothers and 6 sisters" or if you are an only child, you get the really simple answer of "no!".
This question does seem a little long, but simply split it up into three lessons, the first you teach "Do you have ….?" (e.g. do you have a pen?), the next lesson can be basic family words including brother and sister, then in the final lesson put the two parts together to get:
"Do you have any brothers or sisters?". Really easy!