Is swearing by other than Allah, such as by one's blessings or children, considered forbidden or an act of shirk (polytheism), even if the one swearing did not intend shirk and believed in Allah alone?

It is impermissible to swear by other than Allah Almighty, and whoever does so has committed minor shirk (polytheism). If the swearer intends to glorify that by which he swears as he glorifies Allah, then he has committed major shirk, which expels him from the fold of Islam. And whoever's tongue is accustomed to swearing by other than Allah, he must accustom his tongue to saying "Lā ilāha illā Allāh (there is no god but Allah)" if he swears by other than Allah, for the Prophet, peace and blessings be upon him, said: "Whoever swears and says in his oath: 'By al-Lāt and al-ʿUzzā,' let him say: 'Lā ilāha illā Allāh.'" And one must strive against oneself to purify the tongue from swearing by other than Allah Almighty.