I wouldn't bother locking a stroller, I'm not even sure it's allowed since the CM's need to move the strollers sometimes. I never leave my camera or purse on the stroller, but I've left the diaper bag, sweatshirts, etc. and nothing has ever been disturbed.
As for a stroller for an older child, yes, DL is smaller than WDW, but it's still a lot of walking for little legs, especially if you are going to be doing long days. Even if they can keep up, it's nice to be able to walk at your pace sometimes and not theirs, especially when out of the parks and going back to the hotel. I don't think an older child would need to ride in the stroller all the time, but it's nice to have it for when you need it. My dd will be 7 the next time we go to DL and I will probably bring one, just in case. I'm definitely bringing one to WDW next month when I take her for her 6th birthday. I just got back from DL a month ago with my 8yr old DS and while he was fine all through the day, and we did looonnggg days, 13-14hrs, by the time it was time to get back to the hotel, I wished I had a stroller for him! LOL, after all that time, I was wishing I had a stroller for me!