If there are two children and the will states it should be divided equally between the two, and one is the executor, does this give him (the executor)the power to then change this, example take 80% instead? Also, with Unlimited POA, once the person had passed away, the POA has no contol over any assets, correct? In this situation, the POA is not in the will. Does having the POA give him some leverage to get into it?

The executor is charged with distributing the estate according to the terms of the will. They cannot change those terms.

Once the grantor of the POA dies, the POA is no longer in force and cannot be used.

If you believe the executor is acting improperly, then take your suspicions to the probate court and ask for an accounting. If the estate has not been submitted to probate, then you have to force it to be submitted.