Well since your Dad is the dad, you're gonna have to do what he sayes. My son is 13, 5'2", 145 lbs. I bought him a 2003 KX125 for christmas. He has no problem starting/riding it. He came off of a 2003 KX100 that he could flat-foot.
See if you can get Dad to take you to the local shop and sit on a few bikes. Let him know that the suspension will sag some as you use it. This isn't the best way to do it but, you can lower the bike 1-2 inches by raising the forks in the trees and increasing sag.
But the bottom line is, Dad will have the final say. Good luck.
Steve.