This is a great brain teaser! Hmm. I don't think it's as simple as 50:50, because you know there are only two data sets and you know they aren't exclusive of each other due to your prior knowledge.
So........... Thinking through this out loud... there are 2 possible sets of data:
Set a - 1 boy (item 1) + 1 boy (item 2)
Set b - 1 girl (item 3) + 1 boy (item 4)
They have brought one boy with them, so either they have brought item 1, item 2 or item 4.
If they brought item 1, that leaves item 2 at home.
If they brought item 2, that leaves item 1 at home.
If they brought item 4, that leaves item 3 at home.
So that leaves 3 scenarios for the remaining item - boy, boy, girl. So I'd say the possibility of the remaining kid being a boy is 2 out of 3. Wow, my gut feel was 1/3 but my (probably flawed ) maths said otherwise!