Number sense is such a challenge! For some kids it is so natural, and for many it requires a very deliberate approach to help them "see" the math, not just memorize the symbol. I really try to teach my kids (starting with the calendar) to think of teen numbers as 10 + another number. When we are counting objects we always make a group of ten first, and then a group of whatever is left.
Here are a couple of his video songs that my kids are loving!