The 100mm will be more flattering if you have the room to use it. Remember that to get the whole family in focus you will probably need to be stopped down a fair bit from f1.4 or f2.
Re backdrops: depends what they want/expect really (and how nice their house is 🙂 ). There might be a suitable backdrop there but if it’s a small/messy house you might be struggling to find something uncluttered.
While bare flash can be ok It’s pretty cheap/easy to get a shoot through brolly and light stand (and makes you look more pro 😉 ), or a small-ish soft box – try ebay.
I wouldn’t worry about a back light personally but are you happy balancing your flash with the ambient light used as fill? You can work wonders with one flash if so IMO.