The first 19 books in the series were written by Gertrude Chandler Warner, the last of the original 19 being published in 1976, just a few years before her death at age 89.
Beginning in 1991, additional books in the series were written by other authors, with book 145, The Election Day Dilemma, published in 2016. (Hmm….did the Boxcar Children know something about the 2016 Presidential Election??? LOL)
There are also 21 books in a series called The Boxcar Children Specials and a prequel entitled The Boxcar Children Beginning: The Aldens on Fair Meadow Farm, published in 2012.
There is nothing particularly wrong with books 20 onward, but we did notice a bit of a difference in writing style. I much prefer the earlier books myself, but we read many of them, old and new, when my children were younger. They thoroughly enjoyed them, especially my son.