Have had a pool for approx 18 yrs ( 2 houses )..
Kids when young, like swimming.......waterfalls look pretty, but have VERY limited use for kids of all ages..
Ledges are good, but keep them to a minimum...as said , kids like swimming...but when young appreciate somewhere to stand up ....but don,t make the shallow end too shallow, the kids will out grow it pretty quick..
Jets ??.....novelty value, that will not get used much after the 1st season or two...swimming for exercise in a backyard pool is a very rare thing, don't know anybody that does it !!!
Make it deep if you can....as kids get older, they all like diving in ( teenagers will adjust that to bombing !! )
Size is not overly important....they will get used to it even if it's small..
Filter ??...don't like maintenance ??....sand filter....no brainer !!
Salt cell...as said, a reverse polarity one...no need to clean the cell !!
Pump...most name brands....probably all made in China ( or close by )...but don't skimp on size..
I would advise against tile interior, but seeing as your in law does it....get him around in 5 yrs to clean them and replace the ones fallen off !!
This is my average backyard pool, but the small waterfall never gets used, the cartridge filters are a pain to clean 5 times a year, the creepy clawly is great and the 6'6" deep end is good for BIG people to swim in !!