Just did a 2 1/2 week trip to London/Paris in June/July with my husband and DD (14). We did it on our own. We enjoy going at our own pace and having options. It did take a bit of research and planning to help optimize our $.
Flew direct to LHR. Spend few days with a rental car to explore Windsor/Bath/Stonehenge areas. London for 6 days. Eurostar from london to
Disneyland Paris (book as early as u can to get best price). 3 nights at Disneyland with 2 parks 2 days passes. Then took train to Paris for 5 days. Flew back home from CDG)
Most ppl recommend weekdays instead of weekends at Disney, but I went on a weekend. It just worked out better with our schedule/train passes/budget.
We had direct flight into London and 1 stop out of Paris. I prefer nonstops but I choose to do 1 stop instead of backtracking. (As it turned out, airline overbooked our return flight and we moved to a direct flight and got home 4 hr earlier then previously planned. Got lucky!)
I think the more important question for u is April or July. June/July was a bit too hot for us. No way for us to go in the spring because of DD school.