We may have to narrow the "Dover Day" a little to accomodate dinner, meltdowns and some semblance of proper bedtime!
About the Heritage Pass...I know I can purchase them online but I assume that we can just purchase them at our first GBHP property?

No they are not sold at properties - English Trust passes are but these are not nearly so comprehensive as the heritage pass. Buy them online and retrieve them at one place in London or for the same price buy them thru RailEurope - i've bought my last two passes thru BETS (800-441-2387), a RailEurope agent who doesn't charge some of RE's fees - you'll pay about the same here as you would in London. I recently made a detailed analysis of UK prices and $ prices thru RailEurope - some RE passes were a few dollars cheaper even. this depends a bit on exchange rates. But no savings by buying there, but can if don't know.

Correction to the above info: The GBHP is avaiable at SOME covered properties (though not all and I have yet to find a comprehensive list of which ones). Mainly the biggest ones. It can also be purchased at the tourist office at LHR, but not at LGW. It is also available at the main tourist office on Regent St.

So you can buy them in the UK - but it is probably better to get them ahead of time so you have the full color catalog of all the properties and it will help w/ your planning