Total permutations of 6 distinct children in 6 distinct seats: 6!=720
Fix sibling pair i to sit in the same column.
The pair has two siblings s1 and s2→ 2 ways to order them vertically (top-bottom or bottom-top).
There are 3 columns → 3 possible columns for them to occupy.
The remaining 4 children can be permuted in the remaining 4 seats arbitrarily → 4!=244! = 244!=24 ways.
Therefore,
3×2×4!=3×2×24=144
2 ways to order siblings vertically → 2×2=4
3×4×2=24
6×8=48
6!−408=720−408=312
312 ways