You thin the set fruit on trees so they will develop as large as their genetics dictate and to prevent limb damage from too much weight.
If the apples your seeds came from were the same size as the fruits on your seed grown trees, then there is only the weight on the branch issue to deal with thinning for.
If, however, the fruits your seeds came from were larger than your tree's fruits, then you need to thin for fruit size, which means you are going to take at least half of the fruits from every branch so the remaining ones can fully develop in size and flavor.
Redhawk
NOTE:
Most apple trees will set fruit in their third year of growth but orchardist remove all the fruit until the trees are in their seventh year.
This is for tree branch development and it reduces stress on the young tree, so the trunk will thicken and be able to support the main branches in later, heavy bearing years.
Do keep in mind that apples grown from seed are not going to be exactly like the parent fruit in most cases. (some are better and some are worse taste and or keeping wise)
I think it is great that you have succeeded in growing apple trees from seed, in their forever home, their
root systems will be really strong.
If you haven't already, do get some mycorrhizae installed around their
roots by injecting it in liquid form. (I use a metal dowel for this and do my watering in just inside the
drip line.)