naschen
to snack
To eat sweets or other treats, often secretly or between main meals.
Die Kinder dürfen vor dem Abendessen nicht naschen.
The children are not allowed to snack before dinner.

to snack / nibble
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to snack
To eat sweets or other treats, often secretly or between main meals.
Die Kinder dürfen vor dem Abendessen nicht naschen.
The children are not allowed to snack before dinner.

'Naschen' (to snack/nibble) can be used with the preposition 'von' + dative. This emphasizes that you are taking a small part of something larger, for example a piece from a whole cake.
Wer hat von der Torte genascht?
Who has been nibbling at the cake?
In its main meaning, 'naschen' (to snack) is often used without an object or with typical sweets. Common contexts are 'heimlich naschen' (to snack secretly) or 'zwischen den Mahlzeiten naschen' (to snack between meals).
Ich nasche abends gerne ein Stück Schokolade.
I like to snack on a piece of chocolate in the evening.
'Naschen' (to snack) is a very common word in everyday language. It often has an affectionate or harmless connotation, especially when talking about children.
Die Kleine nascht für ihr Leben gern Gummibärchen.
The little girl absolutely loves snacking on gummy bears.
The figurative, often erotic meaning is usually recognizable from a metaphorical context. A typical expression is, for example, 'von der verbotenen Frucht naschen' (to taste the forbidden fruit).
Die Affäre war für beide nur ein kurzes Naschen ohne ernste Absichten.
For both of them, the affair was only a brief taste, with no serious intentions.
to cost / taste
'kosten' and 'naschen' can both have to do with eating, but they do not mean the same thing. 'kosten' means to try a small amount in order to check the flavor. This can be with soup, wine, cake, or any other food. 'naschen' is used when someone eats sweets or small treats, often between meals. The words can be confused because with both, you only eat a little. The difference is: when you 'kostest', you check the taste; when you 'naschst', you feel like having something tasty.

Vor dem Abendessen nascht das Kind heimlich ein Stück Schokolade.
Before dinner, the child secretly snacks on a piece of chocolate.
In der Küche kostet Yara die Soße und gibt noch etwas Salz dazu.
In the kitchen, Yara tastes the sauce and adds a little more salt.