“Happy as a child”做主语“I”的补足语,还可以把它放在后面：I got on the bicycle and headed out onto the streets,as happy as a child.=I was as happy as a child when I got on the bicycle and headed out onto the streets.
请再看例句;My father came home at midnight,tired but happy.
这种句子有个特点,就是可以改为"主语+be+when从句"的句式.