nepo

Say it plainly, because the shape of the word invites a wrong guess: there is no ne- in nepo. Esperanto really does have a productive negative prefix, and a learner who has just met nepre or neniam will try to read this one as not-a-po, which is nonsense in both halves. Po is a live particle in Esperanto, the distributive one in po tri pomoj, and reading nepo as its negation gets you nowhere at all. The root is nep-, one piece, straight out of Latin nepos, and it means what your child's child is.

The gender question is the interesting one. Classically nepo is a grandson, nepino a granddaughter, and genepoj the grandchildren of both sorts. In practice nepo in the plural has long been read as grandchildren generally, and a good deal of modern writing uses the singular the same neutral way, especially in the family-tree sense of the third example above. Both readings are current; if you need to be unambiguous, say nepino or use genepo.

Beyond that the family works exactly as you would expect. Pranepo is a great-grandson, on the same pra- that gives praavo for a great-grandfather, and it stacks as far as your patience holds. The one thing Esperanto did not inherit from Latin is the nephew reading: for that you want nevo, which sits one letter away and catches people out at least once.