No. Chuala mé amhrán iontach - "I heard a wonderful song".
Adjectives can (usually) be used predicatively (tá an t-amhrán go hiontach - "the song is wonderful") or attributively (Is amhrán iontach é - "it is a wonderful song"). When used predicatively, the adjective doesn't change, but when used attributively, it changes to match the gender, case (nominative/genitive) or number (singular/plural). But in the case of iontach, there is no difference between the masculine and femine in the singular, and in some cases the genitive plural is also iontach.
tá an t-amhrán go hiontach - "the song is wonderful"
tá na hamhráin go hiontach - "the songs are wonderful"
is amhrán iontach é - "it is a wonderful song"
is amhráin iontacha iad - "they are wonderful songs*
"She's" can be a contraction of "She has", though it's probably more likely to contracted when there's a "got" in the sentence - "She's got great children". I think that "She's great children" is a bit odd, and probably shouldn't be something that Duolingo is presenting as a correct answer (even if it accepts it as one).
Not really. iontach can mean "strange" in the sense of "unexpected" or "surprising" (is iontaí fírinne ná finscéal - "truth is stranger than fiction") not in the sense of "odd", and the usual interpretation of "She has strange children" would involve the pejorative sense of "strange".
(I can imagine a scenario where someone who generally doesn't like children might use "strange" to describe children who confound his or her expectations by being kind and thoughtful and pleasant to be around, in which case iontach might indeed be appropriate).