Toddler hit newborn in the head

lace 💓💕

My 19 month old keeps accidentally hitting my one week old in the head. This last time, (with a toy) my newborn cried like it hurt her. What should I do? Is her head fragile enough that something like a bonk from a toddler could seriously hurt her? Should I just monitor her myself? I’m sure it’s a common thing with multiple kids, but I’m brand new to this, and I overthink everything 🙃