Well, DD2 has been doing it since she was about 1 and she has a nice little chunk of cash now which I've put in Premium bonds in the hope she wins a million! (She's won £50 so far so I'm still hoping...) She's 6, and she has no idea she gets money for it, she just thinks it's a day out in London. Depending what time of day the job/casting is, and where in London it is, we've gone to lots of free places to make a real day of it - museums, parks etc. I think the main benefit to her has been in confidence, she is quite happy to talk to adults, and confident in new situations. It is easy to do if you're the sort of person that doesn't mind going to stuff at short notice - sometimes you only get 24 hours notice, or even less, but I know exactly what to stick in the bag now, and how to keep it to a minimum, so it's no problem.
Try it and see how it goes! The best bit is when the pics get used and you think, that's my baby! (DD2 is still sneezing over London buses, we saw her pic inside one a couple of weeks ago).
Word of warning - even if you get a job, the pics may never be used as the client decides they'll go with the ones from a different day, or the outfit doesn't come in stock so they don't need the pics etc. DD2's first 2 jobs never saw the light of day (though obviously she was paid for them)
DS sounds like my DD3 - now she's 17 mths she's getting a little more forceful, but she is basically cheerful all the time (unless you take something away from her!).