Does it harm or sadden the deceased child that her mother does not visit her grave regularly because she does not know its exact location?

Matters concerning the deceased in their grave belong to the unseen, known only through sharia (definition) evidence. There is indeed evidence that the dead hear the speech of the living and find comfort in their presence, such as the deceased hearing the footsteps of the pallbearers as they depart, the Prophet Muhammad (peace be upon him) addressing the slain at Badr, and the hadith (definition) about the soul returning to greet one who offers salutations to a deceased person they knew in this world. The inability to pinpoint the exact location of the grave does not cause harm; what is important is to take heed and supplicate for the deceased by visiting the cemetery and praying for your daughter and others. Regular visits are not obligatory, but visiting graves is encouraged.