London-1 night in hotel on Piccadilly line?

South Kensington would be convenient, for both Gatwick Express at Victoria, and Piccadilly Line to Heathrow. Perhaps here ...
http://www.parkinternationalhotel.com/en/

Hi Suzanne,

We stayed here in fall 2016
https://www.bestwestern.co.uk/hotels/best-western-the-cromwell-83871
as a couple. It's very near Gloucester Road tube station, on the Piccadilly Line.

We were lucky and had a choice of room – facing street or garden. We chose garden and it was lovely to be able to look out at that. The compromise was it was a smaller room.

Good luck,

Jill

If you arrive on time at Gatwick, you should be into central London by say 5.15/5.30pm. Leaving out of Heathrow the following day, you will need to be at the airport 3 hours before your flight and it will take say an hour to get there, so you will need to depart at 10.20 from your hotel. My Croatia to the UK journeys usually take 7-8 hours door to door, so you may not feel like doing too much when you arrive!

I wouldn't get too hung up about the Piccadilly line, as this may limit your options. Premier Inn are a chain that offer family rooms in a number of locations - best value is booked c 9 months in advance - the price goes up as the date approaches. Victoria would be a good location to look at - plenty nearby and good transport links, but it will be more expensive as it is closer into the city centre. Chiswick is a good option that is slightly further out and a nice place to stay without the hassle of central London. You would be able to find a good restaurant for your evening meal and take a walk in the park.

What is your budget (in GBP please). UK hotel rooms do not have 2 double beds - you will need a family room.

Jennifer's idea of staying west appeals to me -- it's what I usually do myself. The cost of a cab to airport from there is less as you're closer. You can pre-book local cab. If you are open to that it can make your departure day easier and give you flexibility in your choice of hotel location, freeing you from the need to be close to Tube.
Richmond-upon-Thames, Chiswick, Brentford, Kingston-upon-Thames --These communities all offer the chance to stay close to the river, where you can enjoy a pleasant walk. Look at the map on hotel booking site to pick a hotel near river or other interesting green space.

Rydges in South Kensington has a room that should work. The hotel is a block from the Gloucester tube stop. We stayed there with our adult daughter and the room we selected had two rooms with a door between. I think it was the Chelsea suite.. My daughter had her own TV. The door can remain open yet give the boys some space. We only shared the bathroom. Believe it was room 101. I don't recall the price.