An attached booster takes time, though, right?
There is no such think as an "attached booster". I realize this isn't that fascinating a topic for most people, but there are three kinds of car seats for kids -- (1) infant seats, which are rear-facing, hook into the LATCH-system hooks required in all US cars, and last until the baby is 12 months, (2) car seats, which are front-facing, attach to the LATCH hooks, and usually are used for kids up to about age 4-5, and (3) booster seats, which are basically just seats (some with backs, some without) that you just put into a car so that when the kid puts on the regular seat belt the belt is across their chest and not their neck.
#1 and #2 above require several minutes of installing and uninstalling, and I would never expect Tesla to allow someone to waste time doing so during these short test drives. I loathed dealing with the LATCH seats and am grateful to be done forever with them. A booster (#3), on the other hand, is for older kids (up to age 10) and takes no time at all to throw in (or out) of the back seat before buckling a kid in. Having done this thousands of times over the last several years, I can get a booster seat into a car and a kid strapped in much faster than most every other member of my family can get into a car and strapped in all on their own.
As for liablity etc., it's no different for a kid in a booster than a regular person, other than the fact that in some states it's actually required to have kids at certain ages (or weights) in booster seats, so NOT using a booster seat would actually be putting Tesla at greater legal risk than allowing owners to bring them and put their kids in one.
@Transl8r, my advice is to just show up with the booster seat and just do it -- I don't think they'll have the heart to tell you "no" at that point. Time how long it takes others to get themselves and their guests in and out of the cars, and I'm highly confident that you'll beat their times!
@ckessel, I understand what you are looking for in a test drive (it's very similar to what I'd like to know), but everyone has their own reasons and motivations and things they want to do with their test drive, and no one person's reason is any better than another's. By the time I do mine in late July, it will likely be way too late for me to change anything on my order, yet I'm going to do a ride just for the thrill of it! And, having waited over 3 years, I feel I'm definitely entitled to do so. So if Transl8r or anyone else wants to give their kids a thrill, I've got absolutely no issue with that.