Them is plural. How many children were pulled into the water?
Here are some examples from Wiki where, when the sex is unknown, using ‘their’ or ‘them’, is appropriate:
“Somebody left their umbrella in the office. Could you please let them know where they can get it?”
“My personal rule is to never trust anyone who says that they had a good time in high school.”
“The patient should be told at the outset how much they will be required to pay.”
“But a journalist should not be forced to reveal their sources.”