There are so many fun things you can do by adding letters and numbers to your shoot. The wonderful thing about letters is you can make a collage spelling a word with one or more children. Some of my favorite words to spell are, Dad for Father's Day, Mom for Mother's Day and Love for Valentine's Day. Get creative when working with different numbers of children. For instance when trying to spell dad with two kids, take each kids picture with a D and then both of the kids with the A, or when working with a larger group of kids, lengthen the word to daddy. Consider spelling the child's name or using just their first initial.
Letters are perfect for annual photos, helping mark the age that the child is at. I have a set of white letters shown in these photos that I bought at Hobby Lobby. They are normally $2.99 a letter but often go on sale for half off. Don't want to buy letters? You can make large letters or numbers out of foam core as in the photo below. Trace the letter or number on foam core and simply cut it out. You could do the same with poster board, but it won't be as stable.
Letters and numbers are a great way to personalize photos, telling a story of a child at that particular moment in their lives.