Chain falls between the chainrings of the front derailleur when shifting from the big ring to the small one
This is not normal. Modern bikes should not throw the chain off the rings, either off the outside, inside or between the rings when changing gear. 

The front derailleur is probably set up wrong. Don’t let the bike store staff tell you it’s normal. You have a choice here so just buy a bike that works right from the beginning. 

It's true that you don't want to be cross-chained: on largest chainring and inboard half of cassette or smallest chainring and outboard half of cassette. That puts extra strain and wear on the chain, but the chain should not come off if you forget to follow this rule.