Quite a few put planet!......... never assume a fact to be known by everyone.
I thought briefly that the definition might be more to do with what orbits what - maybe stars orbit the centre of a galaxy whereas planets orbit stars. But that's not always true - what about binary star systems and rogue planets? And anyway, stars and their accompanying planets orbit a point in space which is only nearer to the star because of the greater mass.
Maybe it's not so obvious after all
Google is used by all ages, I'd imagine some younger students googling for planet vs. stars? You gotta learn at some time.
I personally haven't heard of the phrase 'stroller' before - only 'buggy' or 'pushchair'.
It may be worth allowing additional answers that are the top suggestions in Australia, Canada and the UK, with the US version as the 'main' answer, especially where the top suggestion isn't used in at least one of those countries.
Those of us who speak "properly" anyway