Being a step parent 😖😖😖

I LOVE being a step parent to Ava. She has never been my struggle in this relationship. My struggle is with her dad when it comes to her...I took her to a splash park today and a kid pushed her down..she scraped her foot and all her dad keeps saying is well I'll go next time n make sure u don't fall or u should have been watching her. I was watching her!!! I cannot control other people's kids and the boy's parents weren't keeping him from running!!! This isn't the first time he makes me feel like I'm inferior since I'm not her birth parent. I'm worried this will affect parenting our baby boy that's coming in August. Ugh!!!! Plus it seems ten times worse with these hormones!