A poor family living in the woods circa 1500; how did they find husbands and wives for their children?

The average woods-living family would not have been quite as isolated as the family in that movie. Remember that they were explicitly ostracized and thus unable to associate with society. But the average family would have had plenty of opportunities to meet new people.

While you did specify time and place, the answer is going to be much the same for any traditional, rural, Christian community. For the record, my specialty is in Italian society of the same period.

First, obviously, is church. Every Sunday, at least, they would have made the trip to the nearest one. There they would have gotten to know the other children who lived nearby and probably had their first flirtations.

Then there are dozens of holidays and festivals, both Christian and secular (i.e. Harvest, May Day), that would have brought families from the countryside down to the village centers. As opposed to church, these visits would have given young people the chance to run around and socialize with their peers with minimal adult supervision.

Finally, depending on your father's profession, you might accompany him to town on market days or business trips. This way you would meet his associates, friends, or business partners and possibly their children. If you don't manage to find someone on your own, there is a good chance your future spouse will be selected from this pool.

As for marriage, if you are a teenage girl around the age of the film's protagonist you generally have two options: you meet a boy and get your father's approval, or your parents set you up with someone and you approve or disapprove. Despite the common conception that the father's word was law, many parents would have been willing to consider their daughters' opinions. This is especially true in the lower classes, where the stakes of marriage were not as high. That is, while the daughter of a duke or rich merchant may have been basically sold off to forge family alliances, a farmer or fur trapper would have wanted little more than a son in law who was well-raised, polite, and had a promising job, or a daughter-in-law who was attractive, healthy, and well-mannered (i.e. obedient--hate to say it, but that's how people thought back then). Research has found that in the Early Modern Era, love marriages were much more common among the poor than the rich.