Big cities + July/August = high number of tourist....................... but.............
You can plan a journey to diminish the impact of crowds.
For the purpose of an example lets pick staying in London.
Next, put the teens and husband to work investing in their OWN ONLINE research for what is of interest to them. Be sure this is their research and not yours, make them invest into the trip planning. Part of their research must include determining ability to pre purchase on line tickets for preferred times so you can skip the line. This tactic is a great way to make more efficient use of your time without the need to bake in a line during the summer heat.
Add time for researching fairs, concerts, markets and local events occurring during your stay.
Plan on spending time for picnics in the wonderful parks prevalent throughout London.
Task someone with becoming the Prince of Knowledge for learning how to use public transport.
London is a huge menu of opportunities and there are so many interesting places to experience that are well beyond the typical well known tourists sites.
Too hot or encountering rain? Do some interior window shopping within department stores (also great place to use toilets).
Be sure and extend your day into the summer evenings, you will be there during the longest daylight days of the year.
Suggest a train trip for a two night stay in York.
A full day trip to Windsor.
Get out on the River Thames to view another perspective of London.
Highly recommend renting an apartment in lieu of hotels. Benefits is a kitchen, more room to spread out (separate bedrooms) and a great apartment host serves as a liaison for local knowledge. Be sure and query about effectiveness of the AC and how quiet is the environment (think street noise impacts)