I like that term 'free range' and feel I know what you mean. Certainly kids who are mollycoddled, not allowed to play in the dust and dirt are more likely to come down with all sorts of allergies later in life. It is in the 'free ranging' that the natural resistance to most common germs are laid down.
However, I still believe common sense should prevail. There are a lot of deadly infant and young child illnesses such as measles, mumps, whooping cough and the like. These need to be immunized against. These are measures which have reduced the ravages of these killers which once killed off a huge percentage of the young before mass immunization was introduced a 100 years or so back.
But to get back: 'Free range?" Yes. Get the kids to play outside. Let a bit of the sun, wind and rain be part of their life, rather than have them inside dawdling over a Nintendo Box or a computer screen. They'll thank you for it when they grow up.