I already have a PoA (power of attorney) if anything happens to me, they can get their funds back, so I can just add a codicil covering the Private Keys etc - and have those stored in a Safety Deposit Box, so I am not worried about that. I want your opinion on the method I am looking to implement and whether you think its a good option.
I tend to believe you should not hold their key. As other mentioned, your clients only need to make an address and then you can simply tansfer the money there. If you want to make sure they can secure it, I suggest encourage them to educate themselves about bitcoin and etc. That should be able to potect their funds quite well.