My focus seems to be off on my 50mm f/1.8 lens, PLEASE HELP!?

I shoot mainly my kids and keep my lens on auto focus, unless they aren't moving... like when they are sleeping, lol. Anyways when I press the shutter half way to line the little squares line up with where I want the focal point to be and then take the pic, more than half the time it doesn't turn out. Where I had it focused isn't sharp... like today I was trying to take a pic of my daughter sitting in the gr counting acorns, I had the stupid little square over her face and that is the part that ended up being out of focus and the bushes behind her were crystal clear. This is so frustrating. What could I be doing wrong?? I've had my camera and this lens for a year now and haven't had this issue. Could I have broken something?? Hoping it's something simple I'm not thinking of.... Thanks!