Owning property under joint tenancy means that there is a right of suvivorship. This right provides that if any one of the joint tenants dies, the remainder of the property is transferred to the survivors. So, unfortunately under your current form of ownership, you will not be able to transfer your interest in the property to your children upon your death. You will not be able to transfer any interest by your will. Upon your death, your interest in the property will automatically be transferred to the remaining owners.
So, you really have two options.
1. You would have to change the form of ownership from joint tenancy to tenancy in common. Under tenancy in common their is no right of survivorship, and you can transfer your interest upon death. This will require that all current owners agree to execute a new deed naming the owners as tenants in common and not joint tenants.
2. You could sell your interest in the property to the other owners, and then pass the proceeds on to your children upon death. If the other owners do not want to purchase, you can bring a civil lawsuit called an action for partition. You cannot continue to own property with others if you don't wish to. So, in every state there is a civil lawsuit called partition in which a court orders a sale of the property and the proceeds are divided among the owners. Before this happens the court orders an appraisal of the property, and gives the other parties the option to purchase, and if they can't or wont, it will order a judicial sale.
So, under your current ownership form you would have to use one of the above two options to transfer your interest to your children when you die.