V-Flyer

Flying BOS-LHR-DEL, there is a 15 hr layover at LHR
Similarly flying back DEL-LHR-BOS, there is a 20 hr layover at LHR.

Traveling with kids, any suggestions around visiting London for both the trips ?

That's quite some layover, especially on the outbound as it's a short flight from Boston, then a long day in London - what cabin are you flying in? On the return, your layover is overnight, I assume you're booking a hotel?

Not sure what time of day it will be but some suggestions if it is during the day

You could get the tube (Picadilly line direct from Heathrow) to South Kensington and visit the Science museum and the Natural History museum which are practically next door to each other. Both are free and absolutely fantastic for kids of all ages. For a bit more culture, the Victoria and Albert is just across the road.

You are also just a short walk away from Hyde Park & Harrods.