Yes, agree that the current state of gay characters in Western animation leaves something to be desired. Would think if even *Archie Comics* can now have a gay character (to great sales) that a primetime animated series that's supposedly "daring", "edgy," and "no holds barred" (as they all claim to be) are too chicken to show a well-developed gay character (vs ones that're either closeted, some sort of stereotype not seen in live-action since the 70s/80s, and/or just treated as jokes).
And yes, it has to be actually shown/stated the characters are openly gay, with any relationships treated just like any of the other characters'---not the "nudge nudge, wink wink" treatments of Maggie Sawyer on Superman:TAS and other shows, nor stated by the creators long after the show's axed. Simply "strongly implying" they're gay doesn't cut it.
I doubt it'll take 20-30 years to see an openly gay character in kids' shows, if (as noted above) kids can already see gay characters in Archie comics, (live-action) primetime sitcoms, and shows aimed at teens (Degrassi High School), not to mention openly gay celebrities they're already used to seeing (Ellen, probably some MTV person, etc). Like how long it took from the civil rights movement (peaking in the mid-60s) to seeing Valerie in "Josie and the Pussycats" show up in animation as a lead character in the early 70s, perhaps it'll happen sooner than we think (or a lot sooner than 30 years)...
Re: gay characters in animation: I haven't seen "Mission Hill," but that sounds like a positive sign. There's also stop-motion animated "Rick and Steve: The Happiest Gay Couple in the World" on LGBT-oriented cable channel Logo, though that probably tends more toward stereotypes (or mocking them)/being specifically gay-oriented (vs "mainstream").