Ecclesiastes 3:21 “Who knoweth the spirit of man that goeth upward, and the spirit of the beast that goeth downward to the earth?”
When God created man He breathed into Him and gave him life. God’s breath made man a living soul (Genesis 2:7). A soul is eternal, it never stops existing. Nowhere in the Bible does it say that animals are living souls. Man has an eternal soul, animals do not.
In addition, God spoke the animals into existence (Genesis 1:24-25). When God created man, He took the time to form man from the dust of the ground, make Him in His own image, and then breath His own breath into him.
Ecclesiastes 3:21 makes it clear that every man’s spirit goes upward (to God for judgement Hebrews 9:27), but every beast’s spirit goes downward to the ground. The spirits of animals do not go up to God. When an animal dies, they turn back to dust and they stop existing. When a person dies, they go up to God for judgement, and then they continue existing in either heaven or hell for all eternity.
Where a person spends eternity is based on whether or not they have accepted Jesus Christ as their Savior during their life on earth (see https://watchmenonthewallministries.wordpress.com/are-you-going-to-heaven/ to learn how you can accept Jesus). This is one of the things that separates us from the animals. We are granted the privilege of having a personal relationship with God. The animals do not have this privilege.
In addition, if a person dies before they reach the age of accountability (develop an understanding of right from wrong), they do go to heaven. Therefore, all babies and people who are severely mentally handicapped and never understand right from wrong go to heaven (2 Samuel 12:1-23).
However, according to the Scriptures above animals do not go to heaven.
Many parents, teachers, and pastors do not know about Ecclesiastics 3:21 , and they do honestly tell their children that they don’t know, but that they think that animals probably would go to heaven. I had a pastor once who believed that Jesus would put in heaven whatever would make us happy, and therefore our pets would go to heaven. At that time I had not discovered this verse yet, and it sounded reasonable to me.
However when I find that the Bible disagrees with what I have been taught, I abandon what I was taught wrong and change my belief to match the Bible. This is the duty of every Christian.
A parent, teacher, or pastor who knows about Ecclesiastes 3:21 and tells children that their pets will go to heaven is intentionally lying to them. And they will have to answer to God for that.