Hi Karleigh!
Thank you for bringing your question to the Disney Parks Moms Panel!
Typically,
Magic Kingdom Park Guests are allowed to enter the park up to an hour before the park officially opens. Just so you're aware, there is usually a dedicated line for Guests with early dining reservations. If you don't see it, simply ask a Cast Member to point you in the right direction.
While there is nothing "official" about early entry, I would guess that you would be allowed entrance into Magic Kingdom Park no later than 7:30 a.m. If it were me, I would arrive by 7:15 to see whether I could get in then. Since
Seven Dwarfs Mine Train does often begin running before actual park opening, it would be a great idea to go there first to see if you could take a quick ride before your reservation. Again, it may or may not be running, but it's definitely worth a shot!
I hope this helps! Please let me know if you have any other questions.
~Lori