M&S is usually Spring Studios, about a 10 min walk from Kentish Town - we've got a shoot there next week.
Last White Company one DD2 did was at somewhere called "The Roost" - can't remember exactly where it was, but parking was fine there - meter parking, not too bad a rate, plenty of spaces. Don't know if they always use that studio, but they did autumn and winter 2008 there.
Never assume there'll be any food at a shoot - sometimes you will be invited to share if they have a buffet lunch but not always! How old is DS2? Basically, take something to amuse him, and enough food/drink for the day. Shoots don't always run on time so there may be some hanging around. General "etiquette" - you are there to provide a child clean and on time, so don't have long conversations on your mobile and ignore your child destroying the place (I have seen that before!), keep your child away from the shoot area when they are not needed, and generally keep out of the way but be ready to come when called! If DS is little, does he have a toy that almost guarantees smiles? If so, take that if you can.
It is fun, some of the photographers/stylists are really nice and you get to see familiar faces after a while. I've only once had an "off" team (and that was for neither of those companies!). I used to get a bit stressed but now I just chill - they are children, so you can only do your best, if they don't get the right shot on the day, sometimes it can't be helped.
Good luck!