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The field is set up as such:
Opponent has Aerial Responder (2/3), Sun Sentinel (2/2) and Child of Night (2/1) in play.

I have Ravenous Harpy(1/3, +1/+1 counter) and Doomed Dissenter (1/1) in play.

She attacks with all three of her monsters, I declare Doomed Dissenter as a blocker, then use Harpy's effect (1, Sacrifice another creature: put a +1/+1 counter on Ravenous Harpy) to sacrifice the doomed Dissenter. This activates Doomed Dissenter's effect (When Doomed Dissenter dies, create a 2/2 black Zombie creature token), and then I finish declaring my blockers on the other two creatures. If I'm thinking correctly that would mean that Child of Night stays blocked but takes no damage because the creature (Doomed Dissenter) died before combat, Sun Sentinel dies as a result of being blocked by the 2/2 Zombie creature token (who also dies) and Aerial Responder dies as a result of a twice pumped up Harpy.

Was that legal or did I not think about a rule during all that, which would make that play illegal?

Here are the most relevant rules for this question:

509.1 First, the defending player declares blockers. This turn-based action doesn’t use the stack. To declare blockers, the defending player follows the steps below, in order. If at any point during the declaration of blockers, the defending player is unable to comply with any of the steps listed below, the declaration is illegal; the game returns to the moment before the declaration (see rule 721, “Handling Illegal Actions”).

509.4 Fourth, the active player gets priority. (See rule 116, “Timing and Priority.”)

In Magic: The Gathering, unless you have priority, you can't cast spells or activate abilities (aside from mana abilities). Since you don't get priority until after blockers are declared and damage assignment order is determined (second for attackers, third for blockers) it is too late to activate Harpy's ability and still block with the zombie created.

You can however still sacrifice the Dissenter to the Harpy after blockers are declared. The Child of Night will still be blocked, dealing no damage and getting no life link, and the Harpy will still be a 3/4, large enough to kill the Responder. You will have to take the hit from the Sun Sentinel there, unless you have other creatures to block with or other ways to remove it from combat.

No, that sequence of actions is not legal. You declare blockers all at once; you can't declare some blockers, then do something else, then declare other blockers. So, if you want to active the Harpy's ability, you have to either do it before declaring any blockers, or after declaring all of your blockers.