Struggling with effect

I am new to premiere and I am trying to do a video where my kids are running out of the way because the ground is collapsing. I have an effect for the ground collapsing, but at the end when theya re looking at it when they come back on camera the effect is on top of them. I am trying different things with masks but can't figure out how to ensure they are on top of the hole in the gorund. Is there anything out there that explains this process?