Praise be to Allaah.
There is no hadeeth in the Sunnah, as far as we know, to suggest that there is any prohibition on hanging out children’s clothes after sunset. Rather what is forbidden is for children themselves to go out after sunset, in the sense of letting them go out to play and so on.
We have discussed this issue previously on our website in some detail. Please see the answer to question no. 125922 and 127141.
And Allah knows best.