Thanks for the correction.
You weren’t alone being in error. So was I :)
Willie (William) wasn’t the second son but the third. Robert was the oldest. Edward, the second son, died at age 3 on Feb. 1, 1850. Willie, the third son died as mentioned in the WH. Tad (Thomas) lived to the age of 18. I knew the second oldest child had died before the family went to Washington (for the presidency), but I phrased my remark improperly. I wanted to say Willie was the second oldest child at the time, but misspoke. Sorry.