If my children go to bed before Tzeis, how early can I count Sefiras Haomer with them? Is there a difference between preschool age and elementary age?
The Alter Rebbe writes (Siman 489:12) that it is appropriate to wait until Tzeis Hakochevim to count Sefira, however, he mentions that the Minhag of the average person was to rely on the opinion that one can count Sefira from Bein Hashamoshes. If one counts before Bein Hashamoshes, they should count again without a Brocha.
So, if it is difficult for your kids to stay up until Tzeis Hakochevim, they may count after Bein Hashamoshes.