Monday, the day after my step kids went back to their mom's, she calls and says their youngest daughter(2) has a huge…

Monday, the day after my step kids went back to their mom's, she calls and says their youngest daughter(2) has a huge bruise on her butt/leg area and that the daughter told her the daycare teacher did it. when we got them back on Wedneaday, I asked if her teacher had spanked her and she said "yes" I said where and she said "my hand" and showed me then when I asked why she did that, her response was "because I hit her hand first". ok. so then at bed time as I lay her down to sleep she tells me "I have a bruise on my butt" I said "I know where did that come from?" she replies with "(insert step dad's name here) did it." I ask why, she says "I was being bad, I didn't get in my carseat in my mommy's truck". I told her I was sorry she got hurt and gave her a kiss goodnight, then went downstairs and shared this story with her dad. the next morning, Thursday, my SO asked his daughter about her bruise(while I was not in the room but within hearing distance) and he was told THE EXACT same story. On Thursday, I took a picture of the bruise at the end of the day (picture shown) and the bruise is definitely obvious and still there today. we have plans to ask the daycare teacher about it and are just wondering what our options are as to what we can do to document this. any mama's been through this?? we are in Washington State