Re: Travelling to Wimbledon
I'd be very surprised if you had to prove DS's age, especially if he's small! I've taken DH's nieces on the tube a few times and never had any problems, and they're all tall for their age.
Allow a few extra minutes for getting out of Euston underground station - it's right underneath the mainline station, but it can take a good five minutes, occasionally more, to get up to the main concourse as it's such a busy station. The tube gets minging hot in the summer as it's not air conditioned for the most part (outside temperature + 10 degrees is a good guide!) so make sure you've got some water! If it's really busy, stand between the rows of seats rather than by the doors. You won't be quite so squashed in. The end carriages on the trains are usually less busy so more chance of getting a seat!