First, when they're very little, you can't quite use a regular bathtub yet.  I recommend:

These are my favorite washcloths:

And this is my favorite soap:

And if they have dry skin, especially on the scalp, I put a little coconut oil on the little coconut:

Speaking of coconuts, this will protect their head somewhat:

And eventually, toys and books become a fun part of the bath

Throw these trains out when they get moldy, or use a hot glue gun to seal them up: