When the owner of a property is married and has children, parents don't receive anything.
As far as I understood is that Jim is on paper the sole owner of the property. If he passes away, Sue will receive 1/4 ownership of the property and the remaining 3/4 will be shared equally amongst the children.
Like others have pointed out before...a will won't change anything.
If you want to be on the Tapu, you will need to pay the costs involved with a regular property transaction. You can be on the Tapu for 1/2 but you could also choose for 1/4 for you and 1/4 for your wife.
If you chose to be on it for 1/2 and you pass away in the future, your wife will get 1/4 of the 1/2 which means 1/8 of the property. Your children share 3/4 of the 1/2 which means 3/8.
If you choose to be on the tapu for 1/4 each and something happens to you or to your wife, the remaining spouse will receive 1/4 of the 1/4 and with his/her own 1/4 this makes a total of 5/16. The children share 3/16 equally.
Hope I've been able to explain things without making it too confusing.