I am looking for the name of an older sci-fi short story (read about 40 years ago) about genius kids who disappear (possibly into another dimension) and their father feels they ran into trouble there and is worried when they don't return. The story ends with him preparing to attempt follow them and rescue them wherever they might have gone. I read this as part of a Sci-Fi short story collection/anthology in the late 1960's but can't seem to find this story now.
Sounds like "Star, Bright" by Mark Clifton.
The main character has a daughter named Star, a genius, who becomes friends with a boy who moves in next door, also a genius. The two play together, using Time Travel (which the dad eventually learns) via visualization of a different dimensions, but, after he bans them from leaving this time, find a way to go somewhere without technically leaving the present. The father's concern is that they can't get back from where they have gone.
It's available to read for free, online, here.
It's commonly compared to Mimsy Were the Borogroves, but where Mimsy ends with seeing the children vanish, Star Bright ends with the dad working out mentally the steps to follow them, and the story suddenly cutting off (presumably because he has managed it.)