If quitclaim is just simple add son and daughter to deed, is the transfer exempt from the documentary transfer tax?

No, it is a sale of the house in reality from the current owners to the new owners which would be the old owners if they are to stay on the deed plus the new people added as new owners with the old owners.

Also if there is a lien ( mortagage ) on the propertry often you can not do a new deed, since that is against the rules of the mortgage.

In that case, how can I calculate transfer tax? Is it same as property tax?
Also, there is no mortgage or lien on the property.

Transfer taxes are state-set rates. You can probably get your local rate from the county clerk or county register's office, since the transfer tax is usually due when you record the deed.

I suggest that you contact a title company to have this handled. It won't be expensive and they will prepare the deed correctly, calculate any transfer taxes and handle the recording. If you make a mistake, particularly in preparing the deed, it can be very difficult and costly to repair.

Thank you for the answer. I called several title company but they don't do it without open escrow. Do you know anybody in Torrance, CA area?