Regarding crumbs that fall on the floor

I heard that if someone throws bread crumbs on the floor, they will definitely be poor, and it worries me because it is impossible to eat without crumbs falling on the floor, and even if I manage, what do I do with young children?

Greetings,
The prohibition is specifically regarding pieces that are the size of an olive [“kezais”, the size of a box of matches], and only to intentionally throw them in a place where they will become dirty or disgusting or to step on them. There is no issue with what a young child throws on the floor, but if it is a piece the size of an olive, you should pick it up. Of course, if it is possible to teach him to behave respectfully with food, it is advisable to do so.
However, there is a point in not stepping even on smaller pieces, so after eating, it is advisable to sweep the area, especially if small pieces of bread do indeed fall, and particularly if together these pieces do add up to form an olive-sized amount.
In the language of the sages, 'crumbs' does not refer to what is commonly referred to as crumbs in spoken Hebrew, but rather whole pieces of bread. Therefore, even a slice of bread is called a 'crumb' in the language of the sages, and thus there are crumbs that are the size of an olive.

Shulchan Aruch (Chapter 180, Sections 3-4) and Mishnah Berurah (ibid.).