Peace be upon you and God's mercy and blessings.
Ayatollah Khamenei (may God protect him):
There is no issue in managing their funds, but with the conditions of enjoining good, you should advise them to pay the Khums.
Source: Official Website.
Ayatollah Sistani (may God protect him):
It is permissible to manage their funds, and you may engage in all transactions with them, such as buying and selling. In all of this, the reward is for the believer, and the burden is on the one who prevents the Khums.
Source: Official Website: 1, 2.
May you be successful in all good.