No. Czech numbers (including words that define amounts like many, a lot, few, several...) are weird.
2, 3 and 4 are followed by nominative and would have JSOU
5 and everything higher, including undefined amounts are followed by genitiv and JE
What I mean with the nominative and genitive
3 děti jsou modré.
6 dětí je modrých
mnoho dětí je modrých
The TIPS for numbers are on the list of Tips next, so hopefully the explanation there will be little more "scientific"....
I am sorry if my first comment was not clear enough. "There are many children here" is the correct answer in English. "Many children are here" does not make sense from a grammatical point of view. This is the full comment. Moreover, what you just wrote is not true. Doulingo rejected "There are many children here".
"There are many children here" is definitely accepted. But we have no current report, so if that was rejected for you, we can't tell you why; often there's just a typo or other small error. In any case, we've seen a number of instances recently in which Duo rejected valid translations. Perhaps the grading bug that we experienced a year or two ago is back.