Discord has never had a child before and is positively enamored with wee Sunny. Celestia on the other hand has had children before, though it's been many many years; her young son Sparks met a tragic end and for a long time she couldn't bear the thought of having another child if she was going to lose them like that.
Teaching Cadence and Twilight reminded her how much she loved being a mother though, and it wasn't hard for Discord to talk her into it. She and Discord's will they won't they relationship goes back many many centuries, though only recently were they finally able to get it to work out. Celestia's very glad to be surrounded by creatures she loves who are little less susceptible to old age and sickness.
Young Eventide was the first to be born the royal alicorns and she's not sure what to make of these smelly crying followups. She GUESSES they're kind of cute. Sometimes. May as well defend them with every ounce of strength in her tiny body