I'm currently playing Lucky Palms, and several female NPCs have had newborns. I want to play them as children and skip the toddler phase. I know I can grow them up one by one, but is there a more convenient way to add a certain number of days to the town so that all NPCs will age up certain days to make it more realistic?

A couple of ways... Nraas Master Controller, in Intermediate section, you can set age. More immediate, you can take them to CAS, and age up that way.

I wouldn't force age-ups on existing already in-town sims in CAS unless there were absolutely no other alternatives. It won't always go wrong, but there's a fairly good risk that each sim might end up with mismatched body parts -- obvious ones I mean, like a teen with a child's head or misshapen legs. Sometimes they "grow out of it" on their next age-up, sometimes not.

Yes, you want to add to sims' ages by way of MC and not the overall game clock/calendar. There are mods out there that will let you do the latter but they are meant to force time of day (sunlight, shadows, and moonlight) and seasons changes for purposes of taking posed screenshots and maybe brief videos, but never to be played forward.

Messing with the game clock/calendar disrupts the passage of time, thus impacting scheduled tasks and alarms besides birthdays behind the scenes where certain things will miss out on dependent tasks that were meant to come before them. The result if played forward, after a while, is usually a broken game. I'm pretty sure that TS3 is unique among Sims games in having this restriction because it's the only one with the Open World, having households progress through their lives independently of each other while active gameplay is elsewhere in the world.