I am thinking of putting ds2 up in the loft soon. It was always planned to be his room once he was settled at night, and I reckon now is a good time. ds1 is already up there, so it'll be great - a floor for children, a floor for grown ups and a floor for the whole family!
He would be in the eaves room which has a low down velux - it had to be low down for fire regs. Ds1 has been up in the other loft room since before he was 2 and we have never worried about it, but ds2 is a slightly different child!
But ... is it possible to get a lock for a velux window, and even if it is, which is safer - a window that is locked so that toddler fingers can't open it and climb out, or a window that is open so that we can get them out in the case of fire?