They will not pay property gains taxes until (and unless) the sell them
The tax will be a capital gains tax, and if held for more than one year, will be taxed at the lower capital gains rates (as opposed to being taxed as ordinary income)
The new capital gains tax rates for 2013 and future years are as follows:
There is also a new 3.8% medicare tax on top of that (IF their incomes are above ...
Here's an excellent article on that: http://taxes.about.com/od/payroll/a/Medicare-Tax.htm
One lase thing; The capital gains tax is taxed on the gain (net sales price minus the tax basis in the assets) ... If you gift them the stocks, your basis will carry over to them ... if they are left to them at death, then the basis is stepped up to the fair market value as of date of death, so that if they are sold right away, there may be NO capital gains at all