If a Muslim was unaware of what Shirk (associating partners with Allaah) means once while praying and misunderstood a hadith about God loving us like 70 mothers to be the same as Him loving us like children and invoked him as saying that we are like His children by mistake, and then as soon as he learned that the Christians commit Shirk with Him in this way he immediately repented, is it disbelief and Shirk? That person recently heard about Shirk in association and Shirk in love. She might have prayed to Allaah for beauty and attractiveness, not having known that it is a sin indirectly at the time as a norm in supplication, and she might have said at the end of her supplications, "Would you not give it to your children?" Not in particular for that supplication made about beauty, so has she associated beauty and attractiveness with God unknowingly? Now if she applies makeup in public, is it the same as association with God because of the prayer that she made? Or could she be excused for this because of her ignorance? Please, it would be helpful if you could say whether applying makeup and dressing up could lead to apostasy because of the supplication that was made. Please note that the person is also kind of unsure of what she exactly prayed for; it is possible that at the end of her prayers, she might have invoked God in this way for all that she was asking for and not just for beauty.

All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger. 

We did not find any origin for this hadeeth that you referred to at the beginning of your question.

Attributing children to Allah is not permissible; rather, this is a kind of Kufr (disbelief in Allah) because Allah is neither begotten nor did He beget. Allah says (what means): {The Jews say, "Ezra is the son of Allah"; and the Christians say, "The Messiah is the son of Allah." That is their statement from their mouths; they imitate the saying of those who disbelieved [before them]. May Allah destroy them; how are they deluded?} [Quran 9:30]

Therefore, the statement of that woman addressing her Lord with, "We are like your children" or "we are your children" is a grave misdeed.

Nonetheless, not every person who uttered words of disbelief is considered a disbeliever (a non-Muslim), as he might have said that due to misinterpretation or ignorance, for which he is excused. Please review fatwas 8106 and 87963 about the criteria for Takfeer (judging someone as a disbeliever).

This woman asking her Lord for beauty and for being attractive does not result in Shirk, and there is no harm in her wearing makeup and the like if this does not lead to any violation of the Sharia such as being seen with this makeup by non-Mahram (marriageable) men, and the like; Allah says (what means): {…and not expose their adornment except to their husbands, their fathers, their husbands' fathers, their sons, their husbands' sons, their brothers, their brothers' sons, their sisters' sons, their women…} [Quran 24:31] But for this act to be apostasy due to that supplication, then no, it is not. Please read fatwa 336163.

Finally, it should be pointed out that Waswaas (obsessive whispering) regarding the matter of declaring someone to be a non-Muslim is a very serious matter, and one should not resort to it for the slightest reason, because it entails Islamic rulings, such as killing the person (by the ruler) due to his apostasy, separating between the husband and wife, and so on. Therefore, the scholars were very strict about this matter, and they put conditions for it, which we have already mentioned. In addition to this, a person is not ruled to be a non-Muslim due to any act that he does; rather, it must be proven with evidence that what he did is Kufr and that the conditions for declaring him a disbeliever are fulfilled while there are no impediments which prevent from declaring him as such.

For more benefit, please refer to fatwas 323092 and 327421.

Allah knows best.