Roman remains

take a 30 minute train St Pancras to St Albans - Roman remains there

http://www.english-heritage.org.uk/visit/places/roman-wall-of-st-albans/

http://www.stalbansmuseums.org.uk/verulamium/

http://www.gorhamburyestate.co.uk/The-Roman-Theatre

http://www.enjoystalbans.com/attractions/key/detail/24370/verulamium-museum

https://en.wikipedia.org/wiki/Verulamium

You don't even need to leave London!

http://www.historic-uk.com/HistoryMagazine/DestinationsUK/Londons-Roman-City-Wall/

Look at this Historic UK site for maps and info on Roman sites, remains. There are several in or near London. http://www.historic-uk.com/HistoryMagazine/DestinationsUK/RomanSites/ Also consider a visit to the British Museum. You do not mention the age of your granddaughter but she might enjoy reading the book titled "The Mildenhall Treasure" by the famous British author Roald Dahl. She can then see the treasure at the museum.

Take a trip from Coventry to Leicester to the Jewry Wall and associated museum. This is one of the finest examples of Roman building structures to remain in Britain and was part of the wall of the bath house in Roman Leicester (Ratae Coritanorum). The museum contains a number of artefacts directly related to this and elsewhere in the city. St Nicholas Church behind contains material clearly recycled from Roman buildings in its fabric.

I know a bathtub that isn't Roman but behind a window and some people think it is.

An amphitheatre in London? I thought I knew most of the Roman bits in London and that's a new one on me.... Where is it?

There's the London Mithraeum, but I keep reading conflicting reports as to its accessibility.

http://en.wikipedia.org/wiki/London_Mithraeum

I guess its the amphitheatre located under the Guildhall

https://www.cityoflondon.gov.uk/things-to-do/visit-the-city/attractions/guildhall-galleries/Pages/londons-roman-amphitheatre.aspx

Best of both worlds: My mithraeum and Ramblin's ampitheatre and a few more things around and about.

https://www.cityoflondon.gov.uk/things-to-do/visit-the-city/walks/Documents/roads-to-rome_final-accessible.pdf

Don't forget the "source of all wisdom",

https://en.wikipedia.org/wiki/Roman_sites_in_Great_Britain