Well, all I know is that when our son was 16 and I referred to him as an *infant* he NEVER would’ve bought the fact that it’s a British term.
...the correspondence regarding extensions of the period that Lolo was permitted to remain in Hawaii commenced shortly after his study period ran out in 1963, and continued in one form or another for a number of years. To follow all that, you would need to read both the Stanley Ann and Lolo Soetoro material released.
If they married in early 1965, the child referred to would have been not yet four years of age, so if he is referred to as an infant, it would not surpise me.
There’s only one way to be sure. Read the material. Links provided above.