Mtoto, watoto is baby, babies
Mto is river
Mtu, watu is person, people
Mti is tree
Would "The Duolingo Swahili Tree" be "Mti mkiswhaili mDuolingo"?
No. Kiswahili already has a prefix, (the ki-) and you don´t put m-prefix in front of a proper name. Mti wangu wa kiswahili is the way to say it.
That would be "Mti wa kiswahili ya duolingo"
Mti wa Kiswahili wa Duolingo.