This one's easy: Just dress like yourselves!
But since I do actually get asked this question all the time, I send my clients a little 'What to wear' guide in their welcome packs. Generally I suggest you shy away from black, and 'words' or logos on tops, and keep everyone in the family to around 4 colours if you can. Small repetitive prints and patterns generally look fab, as do 'pop' colours in whatever shade you like.
And seriously, as awesome as James looks in these pictures, if your kids have never worn braces and a jaunty hat, I don't recommend the day of your photoshoot as the day to start.
The most important thing to be is yourselves, and to have a great time together during your session.