Why do young children so often mix similar letters or write letters and numbers back to front?
The explanation that I've been taught is that young children see things in 3D. It doesn't matter which way it's flipped or rotated, to them it's still the same letter. They haven't yet learnt to see things flat in 2D on paper. They have yet to learn that when things are written down they remain fixed and cannot be rotated or flipped.
Take the b and d as an example. My prekinders get the b and d mixed up all the time; 'dad' is usually written as 'bad' or 'bab' or 'dab'. Poor ol' dad!