Does it have enough light?
It should grow branches eventually, if you want a tall tree with upper
branches you can live it alone, otherwise cut it down... Not sure if you're
aware, don't expect good apples coming of a seedling tree.

Yes, I'm aware of the fact that we might not get apples at all and if we do, they probably won't be very good. Since my 8 year old grew this all by herself, we couldn't NOT plant it. It's in full sun and we would like it to grow as tall as it can get, there are no other trees near it.

Oh..yea...then live it alone, perhaps stake it.

It sounds like the main purpose in growing your tree is for the tree, not the fruit. So, if you have plenty of room, just let the tree grow. Do prune out dead, diseased, or rubbing branches as you notice them, and consider branch crotch correction for branches with small crotch angles. Other than that, just let it be a tree. If it turns out that your apple tree somehow produces unexpectedly good fruit, you can always start training it for maximum fruit production later (after you've tried the fruit but before the tree gets too big).

One other thing that may come into play is pests and/or disease. You may want to read up a little on fireblight, cedar apple rust, apple scab, and powdery mildew (the four most common apple diseases) and apple pests. Disease and pest problems vary with location, and your local ag extension office may be able to inform you about concerns particular to your area.

Thanks. Yes, we are growing it for a tree and if we get fruit, that's just a plus.