https://www.tandfonline.com/doi/abs/10.1080/01494929.2015.10...
https://journals.lww.com/jrnldbp/Abstract/2016/04000/Same_Se...
When a child has a parent that's died, or a parent that leaves, or a parent that insert 90% of reasons for parental absence, it's natural that they'll have a harder time than anyone else. It's not that the widow would be a bad parent, it's that the kid would have to deal with the terrible situation of having a parent die.
If you were really wanting to get to the source of truth there, though, you'd also show how children that get adopted by a non-couple perform. If I had to guess, they're probably doing the best by far, because adoption as a single person is only really possible if you're incredibly stable.