2,5 week old kitten - hairless chin

Hello everyone. My cat had two kittens 2,5 weeks ago with complicated labor becouse two babies didn´t survive.

Yesterday I noticed that the bigger and stronger kitten has hairless chin which doesn´t look like normal. Can it happen due to sucking too much while eating? The other baby is much smaller and had small eye infection which we are treating with eye drops and it is almost gone.

What is worrying me that the cat and babies are indoors and clean, and supposively the babies are supposed to have elevated defenses due to the fat that they are lactating?

What could be the reason of this chin condition?

Thank you so much for your answers!

Excuse me the mistakes! I´m pretty worried..!

any other symptoms? scratching? 

No, he seems just fine, just eating, sleeping and getting bigger. 

The family has a  problem with fleas, however  not much because I have been very careful in picking them out and bathing the mama cat many times before she delivered (since our vets didn´t suggest using any spot-on), and cleaning the environment.. However since I barely see those bugs, nor the excrements anymore (the mother cat is white), I don´t think it would cause something like this? :O

He looks perfectly normal to me. I've never seen little nursing kittens with fully-furred faces. It ought to grow in as he weans.