What to do after matinee?

Maybe hop on the tube and take a walk in one of the parks of London. Depending upon energy level, could take Rick's "Westminster Walk" which lead to the Thames and Big Ben.

No worries about museums, in London most museums and other indoor attractions close by 5 or 5:30.
For this reason, and to give the kids some exercise, there are many walks outdoors that you can do, in the parks and along the river.
The London Eye is somewhat weather-dependent so you'll want to do that on a good weather day when you can get your money's worth of views.

A walk along the 'South Bank' is always fun, especially on a nice day, loads to see, do and eat. You will walk past HMS Belfast if that's of interest. Pop into Borough Market if you're into food. You will go past the Tate, which is free so you could pop in for 15mins to say you've been, maybe the views from the balcony are the best thing... I'm not an art lover. You will also walk past Shakespeare's Globe, the Millennium Bridge and close to The Shard.

Basically, south of the Thames, from Westminster to Tower Bridge which is about 2.5 miles. You can then walk over Tower Bridge, past the Tower of London and jump on the River Bus (a commuter service, not tourist focused, but still fun, quick and cheaper) back to Westminster. If time (and budget) allows, then jump on the London Eye as the sun sets, London looks amazing as the light fades and all the buildings start to light up.

Actually... that might be a lot in an afternoon :o)

Just a comment: "The Millennium Bridge" mentioned above is the one that is ripped apart in the sixth Harry Potter movie. Might be fun for your boys to be able to say they have walked it :-)

and which some locals, including myself (I know, not London now, but grew up in South London), still call the "Wobbly Bridge" because when built it shook like a jelly.

After the show, a bite to eat, and then off to the Tower of London for the Ceremony Of The Keys!
http://www.hrp.org.uk/TowerOfLondon/WhatsOn/theceremonyofthekeys

The Tower at night is dark and forbidding, seems almost like a completely different place from seeing it in the daytime. The locking up of the tower is cool! Use the website above to reserve tickets right away; they will go fast for the date you will be there, being high tourist season.

Agree, Ceremony of the Keys would be something for all of you to enjoy. 700 plus years of tradition in locking up the Tower. Appears Wicked is playing at the Apollo Victoria theatre. From there you could walk over to the London Eye by walking down Victoria Street past Westminster Abbey and then Big Ben before walking across Westminster Bridge and over to the London Eye. You could also enjoy a longer walk or take the tube to Covent Garden. As sun sets in London it becomes a city of beautiful lights. Have fun.

I wish I had known about ceremony of the keys earlier -it's all sold out! But then again, this trip I only just started planning as it came up kind of unexpectedly, so probably wouldn't have gotten them anyway!

Next time when I have my usual 18 months notice I will put it on the list for sure!

Thanks!
Kim