Customs from US to Rome via London

You'll go through UK border at Heathrow when you arrive. Their customer service at Heathrow is awesome, so it'll be smooth.

You should consider to start heading back to the airport 3 hours before your flight just in case there's a long line at security. It would be awful to miss your flight!

If you have extra time at Heathrow Terminal 5, there's a restaurant called Wagamamas that has good/cheap Asian fusion food.

Also I really recommend buying an advanced seat assignment on the BA website. That way you'll for sure sit together on your long overseas flights.

If you check your bags through, you won't have any contact with the bags or with customs (goods inspection) agents. You will go through passport control in both directions, and will go back through security when you reenter. The Heathrow Express train only takes 15 - 20 minutes travel time from most terminals to Paddington station, but I still round that up and think of it as an hour, including wait time and maybe one Tube connection to your first destination.

If your bags are checked through, they'll be up for inspection at your initial U S landing point.

Thank you all! Erik - we did purchase assigned seats on BA, thank you again for the good recommendations and heads up.

You should plan to be in the boarding lounge at the gate at least 45 minutes before departure, not just at airport security. There is a good chance of another security check at the gate and trans-Atlantic flights often close the cabin door well before the announced departure time.