Hi Nicola! It seems to me that it's fairly natural for numbers to come first.....because we "count" things with children, beginning at a very young age.....10 toes, 10 fingers.....one nose..two eyes.....and it also would seem to me that the concept of numbers/counting comes easily and quickly to a little one.
Even when I read to my children, and now, my grandchildren, I notice that if we count one time with them......the next time, they tend to point at the ducks or puppies and count out loud...."One, two, three..."
I think letters would come a bit later on, when they can perhaps, hold a pencil and even practice forming letters as they learn to recognize and pronounce them.
This is all my own opinion and observations, based on 100 years of child rearing!! LOL......there are probably many opinions on this.... I think mine is fairly sensible...............Peace!!