Is it harmful to bathe babies on Tuesdays and Saturdays?

All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.

There is no harm Islamically in bathing babies or adults on any day or at any time; among the well-known rules by the scholars is that all things are permissible  in principle, unless there is evidence that something is forbidden.

Ibn Taymiyyah said in Iqtidhaa’ As-Siraat Al-Mustaqeem, "The basic principle concerning acts of worship is that nothing can be legislated from it except what Allah has legislated (ordained), and the basic principle concerning customs and habits is that nothing of it is forbidden except what Allah has forbidden.”

Allah knows best.