If babies have a high heart rate why do they not have heart failure if their heart is working so hard?

Heart rate per circulation time is the same in all animals. Smaller animals have higher heart rates than larger ones. The number of heart beats for one circulation of blood is not higher in babies. For a technical publication, visit this site: https://pubmed.ncbi.nlm.nih.gov/8184772/ A blue whale's heart beat, is 9-10 beats per minute while the Etruscan tree shew, has a heart rate of over 1000 .

Because the normal heart is controlled by many things as its goal is to get blood to where it needs to go. Growing requires an increased blood flow to everything. This can be achieved best by speeding up the rate to meed the body needs. So what is high in a adult is normal for a baby. You are making the error of comparing apples to oranges.