My brother asks on behalf of his two small sons, age 6 and almost 3, (my darling nephews) is it permissible to begin the Pesach seder early, that is, before sundown, so that the children will be able to stay up for its entirety?
Unfortunately for your darling nephews it is not possible to begin the Seder before nightfall. But have everything ready so you can begin right at nightfall. Don't rush the proceedings, but do move through them efficiently without wasting time. I also suggest you give the children a nap in the afternoon so that they will be fully awake and able to participate!
Shulchan Aruch Orach Chaim 472