Reducing der/die/das to "de" is at best dialectal thing (but see the more differentiated view outlined below). It is not common in standard pronunciation, also, and notably so, not in sloppy pronunciation.
Also, even where it occurs, it has nothing to do with speakers not knowing what article to put there. Easy prove: even children growing up in dialect-only regions are perfectly able to use the correct articles (i.e. genus of the nouns) in writing. So they know what genus a noun has, even though they may not differentiate the articles in pronunciation (or alledgedly so, because perhaps they anyway do, but not necessarily in a way intelligible to non-members of that dialectal group).
Exceptions in terms of the one or the other child having general difficulties with writing, or simply with German language, are not relevant here. The group we are looking at are average people with German as first language. It is possible that some adults who learned German as an additional language resort to that method to hide their insecuirities.
Interestingly the topic of replacing der/die/das by only "de" has been discussed broadly in the media as a result of a book
Abbas Khider: Deutsch für alle. Das endgültige Lehrbuch
having become a best-seller recently (spring 2019) and getting many book reviews in the media. The author, who once had come as a refugee from Iraq and struggled a lot with learning German, submits a number of suggestions how to make German more easy to use for foreigners, and one of his suggestions is reducing der/die/das to simnply "de".
Checking dialectal use of articles
In Swabian dialect you may hear
Gib mer mol da Hammer! (Gib mir mal den Hammer)
Dr Hammer isch it dô. (Der Hammer ist nicht da.)
Hosch da Soich scho nausgfirat? (Hast du die Gülle bereits auf die Felder verbracht?)
Hosch dr Mariluise ihre Kender gsäa? - (Hast du Marie-Louises Kinder gesehen?; wörtlich: "der Marie-Louise ihre Kinder")
Kasch mol d'Epfl raholla? (Kannst du mal die Äpfel runterholen?)
Kasch mol dui Epfl do brocka? (Kannst du mal jene Äpfel dort pflücken?)
Breng au mol s Gaddadirle zom Richta! (Bring doch mal bei Gelegenheit das Gartentor zur Reparatur)
Which indicates that even in (this) dialect there is no actual redcation of der/die/das to "de". Rather all forms of standard German have their separate pronunciation in dialect: der becomes "dr" (but accusative den becomse da), die becomes d, and das becomes s or as.
Note that vowels a and e in the above examples (in de/da) are rather Schwa, not clear e or a.