This is an important question and the answer has many details.
In the laws pertaining to blessings, there is a rule that a blessing is no longer relevant if the person eating the particular food or drinking the drink has moved from the place or room where he started, to a different area. This means that a new blessing must be said in the new place, as it is considered that once a person has moved to a different place he has also become distracted from what he was doing in the previous place.
If the park or orchard, for example, is enclosed by some sort of fence or enclosure, then the whole area is considered as one place, for the purpose of the blessing. If the area is open, then as long as the picnic table can be seen it can be considered the same area. If people move let’s say, to a play area, and wish to continue eating there, a new blessing needs to be made on food and drink.
If people return to the picnic table from another area and wish to continue eating, then it depends on which after-brocho (blessing after food) is required. If the after-brocho is ‘Borei Nefashot’ a new blessing needs to be made. If however, the after-brocho required is ‘M’ein haShalosh’ (Al haMichya) or Birkat haMazon after bread, a new blessing is not needed because the intermission was temporary. (Admor HaZakein,178:8)
If one of the family members is constantly near the table, no new blessings need to be made, as it is considered that one of the group who is partaking of the food was left in the area and therefore it is as if the others did not leave the place of the meal.