Rabbi Baruch Epstein offers an explanation of why the Cherubs had the faces of infants: it was to arouse God's pity upon the Jewish people. He brings in the following quote from Joshua:
"When Israel was a child, I loved him. Moses said to Joshua, 'This nation which I am handing over to you still consists of young goats, mere infants. Do not be angry at them for what they do, for even God did not become angry at them." (Joshua 11:1)