You can get the tax deductions for your savings today (by giving the amounts to an intermediary charitable fund), the balance will grow tax free, and you can then direct the fund to make a donation to your ward when the time comes to pay for your boys' missions.

http://www.fidelitycharitable.org/giving-strategies/give/donor-advised.shtml

In essence, you create your own "charity" that you can donate to in small increments over time. You get the tax deductions now, but you lose ownership of the money. (Technically, Fidelity or the fund owner would own the money.) The fund allows you - as its only donor - to direct its giving in the future, so you can tell the fund where to donate (as long as it's to a recognized charitable organization).

If something were to happen where your boys don't serve, you can still use the fund to pay tithing or make any other donations for you, so you don't lose anything (as long as you plan to give regularly to the church or another charitable organization in the future).

Remember, however, that you will be taking the tax deductions now and will not receive any deductions in the future.