Ear "buds" are harder for young kids to use and put in their ear properly.
You can opt to get the kind of ear phones that are bigger. Which is easier to use.
BUT with young kids, they do not know how... to calibrate the "volume" on their gadget properly, and often times they make the volume too, loud, from their ear phones. Hence, it is bad for their ears/hearing in the long term and will affect their hearing in the long term.
With my son who is 7, we don't allow him to use ear phones, because: he tends to make the volume on his Nook, too, loud. Even if he does not intend to. I can hear it, even if he had ear phones on.
So, he just watches movies without, ear phones. Which is good enough.
Just Google search "ear phones for kids."
And, it also depends on how much you want to spend.