@molliere I do know of one cause, which is an issue with milestones. More specifically, milestones are attached to the object on which they were achieved, and if that object is custom content, you can have an issue if you remove the cc or if it breaks in a future patch. The thing is, this particular issue normally creates a lastexception with a very particular error in it. Still, you can try adding the milestones fix here:
https://www.patreon.com/posts/mod-milestone-81237568
Other than that, I don't know of an issue that would break toddlers and also not be fixed by removing your mods and custom content. But it's worth adding back your mods, a few at a time; except replace them all with the most updated versions you can find. If you're not sure which version you have, download a new copy.