omg, i JUST POSTED THIS EXACT SAME THING ONLY i HAVE A 15 MO OLD BOY BITING MY 4 YEAR OLD son. My 15 mo old is getting 8 teeth at once- whew!!! this is really hard. I have redirected also and given him alternatives as soon as I say biting hurts we don't bite! He then has a smirk on his face and my 4 year old starts to cry. This is constant when they are together, but they used to play together nicely... I hope this is just a phase during the teething. Offering many hugs to you and your little one..... I'm sure it can be equally as frustrating for them.