I don't follow the morphology. So first, we can tell it is فَـ plus ترضّن. However, رَضِيَ could not have a ن here. A quick check for the masdar مصدر of وضي doesn't have any that look like that with ضّ or ن. If the verb was رضّى, then the correct مصدر is ترضية. The form تَرَضّى means to try to please, which in addition to not making sense, can't have a ن like that. The verb رضن does not exist as far as I can tell. If I had to guess, I would say this is a typo. Using a present tense verb here with a ت for conjugation seems really wrong. The تفعّل form doesn't work in meaning or conjugation. It seems like they should have been going for رَضَّت "it satisfied [the prince]", or maybe just وضي "he was satisfied". How they ended up with a ن... I don't know. Every conjugation requires ين ending when a ن is used with a so-called ي defective verb (final radical is ي).

فترضّن رَضَنَ: فعل معناه: تقارب خطوه في عدوه In English It's means he started to step out (walk slowly)

Before you answer, I read the original story in French, and I found that (ترضن الامير الصغير و قال ....) translated from: Then the little prince remarked gravement: Never mind, it's so small, at home! but the verb (ترَضَّن) is almost nonexistent. the word that I have found is similar (رضن) but it means: assemble things to each other. or accumulate things. so I try to replace the letter (ض) by (ص), which will give us the verb (رصن) Then I found the following sentence :( رَصُنَ الشَّخصُ) means that the person has become seriously. The adjective (رَصينٌ) mean:Person who is wise. then (رَصُنَ الأمير) explains well the sentence: (Then the little prince remarked gravement ...)

رضى : satisfy يرضى ترضى ترضُّن " نون النسوة"

so/then , a little prince had satisfied , and said : no worries about it ( no problem ) because my homeland extremely small ... satisfied = رضي -- satisfy = رضى and as a noun : satisfaction = الرضا