Here are some ideas. In order to make the opposing side not look like jerks, give them small reasons throughout the episode to be wary of the changeling, for example, the changeling bites or "sucks the love" out of pinkie pie, and pinkie pie becomes more and more like pinkamena diana pie.
If spike was used, the above idea would still be applicable. Instead of slowly becoming pinkamena, spike would slowly become his "greedy" adolescent self. However, Twilight changes him back with magic before he hurts anyone, but he still protects the baby.
another idea is if the parents of the changeling show up and threaten the mane six to give back the baby, however the baby is already attached to pinkie pie.
or, you could have the baby be dangerous for real, but give him/her a special quality that makes the baby invaluable to the kingdom. Like, it could reveal study into changeling magic, or a completely new type of magic, or it was a baby foretold or something like that.
Or, you could have it where Celestia wants to have spike raise it to become an ambassador to the changeling kingdom, and to raise it as a pony. Sort of like a tarzan kind of thing, but obviously not with gorillas