kinderleicht
easy as pie
Extremely easy to do or understand; effortless.
Mit der neuen Software ist die Bildbearbeitung kinderleicht.
With the new software, image editing is easy as pie.

easy as pie
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easy as pie
Extremely easy to do or understand; effortless.
Mit der neuen Software ist die Bildbearbeitung kinderleicht.
With the new software, image editing is easy as pie.

The word is made up of 'Kinder' (children) and 'leicht' (easy/light). The idea behind it is that something is so easy that even a child could understand or do it.
Die Anleitung ist so geschrieben, dass die Montage kinderleicht ist.
The instructions are written in such a way that assembly is easy as pie.
'Kinderleicht' (easy as pie) is very often used as a predicative adjective after verbs like 'sein' (to be), 'werden' (to become), or 'scheinen' (to seem). It can also serve as an adverb to describe a verb.
Das neue Programm zu installieren, war kinderleicht.
Installing the new program was easy as pie.
This word belongs to everyday language and is frequently used in advertising or instruction manuals. It has a positive, persuasive, and slightly informal sound.
Mit unserem neuen Kochbuch wird jedes Gericht kinderleicht!
With our new cookbook, every dish becomes easy as pie!
simple / just
'einfach' and 'kinderleicht' both describe something that is not difficult. 'einfach' is neutral and can be used for tasks, explanations, routes, or rules. 'kinderleicht' is stronger: it means that something is really very easy and takes almost no effort. The word often sounds somewhat casual and emphasizes that there is no need to worry. If you simply want to say objectively that something is not complicated, 'einfach' fits. If you want to show that it is extremely easy, 'kinderleicht' fits.

Mit dieser Anleitung ist das Regalbauen kinderleicht.
With these instructions, building the shelf is easy as pie.
Die App ist einfach zu bedienen, auch wenn man sie zum ersten Mal öffnet.
The app is easy to use, even when you open it for the first time.