Yeast infection (candida). It’s not caused by chewing on same toys as infected kid. It’s just an abundance of this naturally occurring yeast. It is very common, and does not appear to have a medical “cause”.
Anyway, it’s not a sign of “laziness and neglect”.
Note: Make sure you’re daughter/daughter-in-law is aware of how to treat this. If she’s breastfeeding, she’ll need to make sure they’re not passing this back and forth. Thrush can be a bit challenging to treat if only the baby is treated. It can also cause severe pain for the mom if she is infected.