You can create a wallet address for them on a hardware wallet or airgapped device and send Bitcoins regularly to it. You can also intimate your spouse of your plan and teach them how to work around cryptocurrencies if they are not savvy, so they can always access the funds should anything happen to you and still fulfill the purpose.
I'll be depositing funds in their accounts regularly, which will keep sitting there for about 6-10 years, so I'll be looking to accumulate some sort of interest during that period. What options should I consider for this?
This would require some sort of staking on a centralized platform. This comes with the risks that you are not in control of the funds and the platform can fold, seize or freeze the funds or get hacked.
If you're going to invest in a decentralized asset, you should not shirk the responsibility that comes with safeguarding the assets.