The most common are доченька/ сыночек but parents like to use these as well: 1) малыш 2) солнышко 3) зайчик 4) чадо

малыш(m)/малышка(f) - stylistically neutral words доченька - only for your daughter сыночек - only for your son There are also many words that initially have another meanings but can be used like terms of endearment for children. Usually they mean something little, cute or pleasant. They all are informal and metaphorical. For example: бусинка(small bead), пуговка (small button), зайка (little bunny), солнышко (it refers to "sun" but means something like"sweet pie"), лапочка/лапуся, мася/масечка(sweetie). My fiend calls her 4 y.o. daughter "ласточка"(literally it means "swallow" but can be translated like "babycakes"). Here are more tender words for kids: https://www.babyblog.ru/community/post/babyenglish/1726424