If your child is a toddler or preschooler, present her with the popular birthday math question, "How old are you?" Instruct her to hold up her fingers to indicate her age while she answers your question verbally, and practice this with her in the weeks and days before her birthday so she'll become familiar with her new age. Add more questions to help her learn numbers, like "How old were you last year?" or "How old will you be next year?" This helps to reinforce the succession of numbers. Set up a number chart in her room or the kitchen so she can point to the numbers that coordinate with the answers to your questions.
Sit down with your child to help him put together a guest list for his birthday party. Give him guidelines indicating how many people he can invite to his celebration by telling him something like, "Divide 20 by 4, then add 3. This is the number of guests allowed at your party." This helps to remind your child of the division concepts he may be learning in math class, and helps him to remember the basic principles of addition. For younger children, use visual aids to make your birthday math questions effective. For instance, cut out shapes of chairs from paper and present your child with the chair shapes and a small poster board. Tell him he can invite five friends over for birthday cake, then instruct him to place five chairs on the poster board. Work with him until he gets the answer correct, then help him form the letters to write the names of the people on his guest list.
It's best to send birthday invitations to guests at least three weeks before the party. Turn this into a math problem by telling your child she has 21 to 28 days to mail her invitations to her friends. Give her a calendar and work with her to remind her how many days are in a week, and how many weeks are in a month. Once guests start confirming their attendance with an RSVP, help her subtract the number of guests that respond from the total number of people she's invited to figure out how many people still need to respond.
Let your child help you put together mini party favor bags for the kids that come to the birthday party. Purchase three or four items like giant cookies, mini toy cars or kazoos to place in the bag, and turn the activity into a fun math lesson by asking your child how many treats go in each bag, and how many treats there are overall. This helps to reinforce the concepts of division and multiplication as you explain that if there are three items in each bag, and there are seven bags, that means you'll need 21 items total. Help your little one count the items in each bag one last time to ensure that each young guest goes home with the same number of goodies.