Many times I have had clients ask about having a child named as a Joint Tennant with right of survivorship on an investment or banking account. The reasons the client cites is usually to provide liquidity for the estate, avoid probate, and have someone who can access funds should they not be able to write checks for a period of time. I will generally discourage this as it can cause problems with the final disposition of assets and leave the clients vulnerable to divorce or other legal proceedings.
An often overlooked tool is called Transfer on Death (TOD). Many states have adopted laws that enable individuals to transfer assets by contract rather than by will, which greatly simplifies final distributions for heirs. For a complete list of states that have adopted the Uniform TOD Securities Registration Act you can look here.
A TOD account allows you to specify beneficiaries for each account you may have that is registered in your name. You may specify a different percentage ownership for each beneficiary. Upon your death the assets in the TOD account will transfer to the named beneficiaries without the delay of probate, and separate from other items in your will. You can make changes to the beneficiaries and their percentage participation whenever you choose. There is usually no additional charge for having the TOD designation added to your account, but not all institutions may offer this type of account, so be sure to ask.
For banking accounts you should know about the Pay on Death designation which works in a similar fashion, and also avoids probate. Again, ask your banking institution if they offer this account.
If you need someone who can write checks and pay bills for you if you become incapacitated you should have an attorney draw up a Durable Power of Attorney.