I bambini saltano nel fango.

Italian generally uses the definite article with nouns even in general statements. So where English says Children jump in the mud, Italian needs i bambini.
Omitting the article (Bambini saltano nel fango) would sound abrupt or like a headline rather than a normal sentence.

• i is the masculine plural article used before most consonant sounds (e.g. i ragazzi, i libri, i bambini, because b is a plain consonant).
• gli is also masculine plural, but it appears before words starting with a vowel (e.g. gli amici) or special clusters (z, s + consonant, gn, ps, x, y).
  Use i before a regular consonant and gli before a vowel or those specific consonant clusters.

• nel is the contraction of in + il, because fango is masculine singular:
  in + il fango → nel fango
• Italian typically uses the definite article with mass nouns (like fango), so you generally cannot drop the article and say in fango.

fango is treated as a mass (uncountable) noun in Italian, just like English mud. You don't pluralize it for multiple bits of mud.
If you want to talk about separate muddy puddles, you'd use pozzanghere (“puddles”), not fanghi.

Italian uses stare + gerundio to emphasize an action in progress:
I bambini stanno saltando nel fango.
Literally “the children are jumping in the mud.”
Otherwise, the simple present (I bambini saltano nel fango) can serve both habitual and progressive uses, depending on context.