If baptism is required for entrance into the kingdom of God, why is it not required of little children or people with mental disabilities? I understand that because they are without sin, is the reason but so was Christ without sin. Should it not be required of them in the same as a required ordinance of Christ who was perfect?
The problem here is a universal application of the idea that all must be baptized. Through modern-day revelation we can understand this as a qualified statement with the implied words “who are accountable” added to it. All who are accountable must be baptized to enter into the kingdom of heaven. Those who are not accountable (little children) do not need baptism. For “it is solemn mockery before God, that ye should baptize little children.” (Moroni 8:9)
If we understand John 3:5 (Except a man be born of water and of the Spirit, he cannot enter into the kingdom of God) as a face-value, end-all statement, then this could be confusing. But latter-day revelation and the Book of Mormon have clarified these things for us.
For example, Moroni 8:10:
“Behold I say unto you that this thing shall ye teach—repentance and baptism unto those who are accountable and capable of committing sin; yea, teach parents that they must repent and be baptized, and humble themselves as their little children, and they shall all be saved with their little children.” (emphasis mine.)
And from D&C 29:46-47
46 But behold, I say unto you, that little children are redeemed from the foundation of the world through mine Only Begotten;
47 Wherefore, they cannot sin, for power is not given unto Satan to tempt little children, until they begin to become accountable before me;” (emphasis mine.)
I suggest you read this article by Bruce R. McConkie that explains latter-day revelation and clarifies our understanding of child baptism, including thoughts such as:
“Jesus loves and blesses children. They are the companions of angels. They shall be saved. Of such is the kingdom of heaven.”
I would also give Moroni Chapter 8 a thorough read-through.
Therefore, your question is being asked on a false premise — “If baptism is required for entrance into the kingdom of God….” The reality is that it is not required for all, only those who are accountable, and that is the simple and plain explanation.