Why does evo. psych imply this result? The fact that you can spawn healthy children may make you more attractive to an additional potential mate, but the potential mate must also consider whether you can provide resources to support additional children.

more people will share resources to help my kids survive. (emphasis added)

This seems like the opposite of what evo. psych would predict. Your relatives might be willing to provide you more support if you have more kids, but why would strangers be more willing to support you based on your reproductive fitness? As for relatives of additional potential spouses, the considerations of I discussed above still apply.