Great pics Sharn! Those boys sure are growing like weeds. I wouldn't worry about the climbing on the back thing. Fillies do it too sometimes. I think it's just a stage that all foals go through when learning how to play. The first thing they have to play with is Mom.
Speaking of climbing on backs, the cows in the adjoining pasture are humping each other like mad. And that's the funny part, they're COWS humping other COWS, not bulls. And they're making all these really strange noises. It's like a cow orgy out there! I'm glad I was by myself or it'd be embarrassing.
Yes, climbing up on mom or anyone else, including you, is a very normal thing... so beware, the little guys will at least try to get ya!