Hi, Ann! You can certainly purchase your theme park tickets prior to your vacation. In fact, I recommend it to save time and to get all your travel plans in order.
The first place you will want to check is
HERE. You will see the different ticket options and pricing. Since your child is 10, he/she is considered an adult so you will want to price out 3 adult tickets. The more days you purchase, the more the "per day" cost of the ticket drops. Once you get to a 4 day Magic My Way ticket, you will see it costs very little to add on additional days. So if I were you, I would opt for a multi-day ticket.
You can also purchase tickets at your local Disney Store. To purchase tickets at a slight discount, check-out
www.undercovertourist.com or your local AAA office.
Plan out your vacation and figure out how many days of tickets you will need before purchasing. Also, figure out if you want to add options such as park hopping, water parks, or no-expiration. After doing some research, let us know if you have any questions. The Moms Panel is here to help!