What is the Biblical Protestant teaching on death of children and infants in Christianity?

No Scriptures support this Children are a gift of God so one doubts if God would condemn his gift also every grown up person in his life time has a chance to repent and be Baptised repentance requires confession to God and a turning away from sin CAN a child do that and How can a child know good from evil ?

I don't know what the Protestant teaching is. According to God's Word, currently, all souls that leave these flesh bodies go to God and the flesh bodies return to the earth minerals they are made of. The Lord's Day is a thousand years, (2 peter 3:8 KJV) of teaching God's Word while satan is bound and unable to interfere. We will all be in our spirit bodies at that time. Some will have already overcome through Jesus Christ, the 'second death' which is death of the soul and some will still be mortal during that teaching time. Family members that have overcome through Jesus Christ will be able to help loved ones who have not overcome, yet. Then, after the thousand years, satan will be released to test the mortal souls. Revelation 20 KJV We do not know all the experiences and thoughts of other people, even our own children, so we cannot judge their soul. God only is Judge of souls. James 4:12 KJV Attempting to take God's place of judging souls is satan's sin.

Ecclesiastes 12:7 KJV Then shall the dust return to the earth as it was: and the spirit shall return unto God who gave it.

Matthew 18:2 KJV And Jesus called a little child unto him, and set him in the midst of them, 3 And said, Verily I say unto you, Except ye be converted, and become as little children, ye shall not enter into the kingdom of heaven. 4 Whosoever therefore shall humble himself as this little child, the same is greatest in the kingdom of heaven. 5 And whoso shall receive one such little child in my name receiveth me. 6 But whoso shall offend one of these little ones which believe in me, it were better for him that a millstone were hanged about his neck, and that he were drowned in the depth of the sea.

Matthew 19:13 KJV Then were there brought unto him little children, that he should put his hands on them, and pray: and the disciples rebuked them. 14 But Jesus said, Suffer little children, and forbid them not, to come unto me: for of such is the kingdom of heaven.

There is not a single Protestant teaching regarding this subject.

Personally, I've never heard the teaching you describe. Those Protestant sects with which I am familiar that differ from Catholic teaching teach that children who die young are forgiven their sins until they reach "the age of accountability" (which age varies widely from Protestant sect to Protestant sect).

According to the Book of Mormon, children are saved.