It's a common thing for kids to go through. My kids went through it about a year and more out of frustration. Honestly, the worst part is for us parents rather than the kids!
The big thing is to understand it's just a phase, don't give attention to it. I know it's SO hard to not wince or run up to them, but my kids thrived on the OWIE attention, so I had to ignore it. Kids' heads are pretty tough and it's generally the front part of their head which is made for preventing trauma.
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