By no means am I an expert, but if you can’t spare the $.97 for a three pack of dial anitbacterial (or perhaps the time to go to the store) I would think it’s not the end of the world. The point of the soap is to keep it clean and free of infection with as little irritation as possible. You have an open wound and that skin is going to be more prone to infection and allergic reactions. An allergic reaction can cause you to be mucho uncomfortable and may cause blemishes that will damage your tattoo during healing.

Keep it clean, and if you can use the baby wash (I would expect it to be pretty mild) just watch for signs of irritation. I hope, of course, that you’ve already remedied this situation by now, though.