In order to receive the Body and Blood of Christ, the person has to believe it is literally His Body and Blood. If they do not, it is a grave sin (all other issues of culpability aside).
With this in mind, I think from the earliest age, we should be telling our children the Truth in it’s full beauty and amazing Salvation. “The bread becomes the Heart of God, His Flesh, and the wine becomes the Blood of God.” If they ask “is it really His blood?” We simply say “yes. It is the Blood of God, the Creator of all that exists. He loves you that much, little (boy or girl), that He gave you His Heart and His Blood for you to go to Heaven forever and ever!”
The reason I say to tell them it is His Heart is because miracles have confirmed that the flesh is literally His Heart!
I think most kids know what “heart” means and know what “blood” means.
No detailed explanation of transubstantion needed. Tell them what they are going to consume, hopefully, for all of their journey and their fight here on Earth against Satan and his angels.
From Revelation 12
…10Then I heard a loud voice in heaven, saying, "Now the salvation, and the power, and the kingdom of our God and the authority of His Christ have come, for the accuser of our brethren has been thrown down, he who accuses them before our God day and night. 11"And they overcame him because of the blood of the Lamb and because of the word of their testimony, and they did not love their life even when faced with death.
yes, I think so too.