We will have to assume that they are both EXACTLY 12 and 17 today (they both had birthdays today)
then we have 10,000 invested at 2% per quarter and that 1/2 of the amount will be distributed in 16 quarters and the remaining will be distributed in 36 quarters and the amounts can be different????? Well there is a whole bunch of possibilites depending on how it is split when the first kid turns 21 !
Wouldn't it be better to split the accumulated amount EQUALLY when the first kid turns 21 and then let the remainder continue to grow until final disbursement to the second child when she/he turns 21 ???? We'll need some more info to proceed.....BUT here is the way I would do it
10000(1+.02)^16 = 13727.86 when the 17 y/o turns 21......give half of the money to them $6863.93
6863.93 will continue to grow for 20 more quarters before being disbursed to the younger child
6863.93(1.02)^20 = 10199.44 to the younger child when she/he turns 21
This MAY seem unfair, but it is NOT....the FIRST child could leave the money deposited and get the same amount if she/he waits until the younger turns 21 to receive the money.