I thought lavender essential oil was safe to use undiluted actually, we use it on scars in our family.
And I don't understand how it could have burnt the bottom of your bath, doesn't oil float? 
As for the cheeky wipes, we use them, the oils make a thin film over the top of the water, we stir it, and then put the wipes in straight away - the oil seems to be evenly distributed! No burns, both baby and I have quite sensitive skin, baby couldn't even wear disposable nappies when he was born. 🤷🏼♀️