Star Trek is fun, colorful, rewarding, and has both easy to understand challenges and some deeper play. ST all the way for a family pin.

St premium all the way. I was not too fond of the TWD gameplay and Star Trek is a great compliment to Metallica. I have an ST premium and it is a great game.

ST for family. To me TWD is similar to Met, so in a small collection would be better off with ST.

I have a TZ next to Met Prem and jump between the two a lot.

Have you thought about a Woz, very family friendly.

ST definitely better for kids.

Easily TWD premium. Very different layout than Met and equal, if not better gameplay.

Between TWD, Met, and ST I don't think you can go wrong, but I would think TWD is a little dark/gory for most families - there are heads getting blown off with shotguns on the DMD, for example. ST would be a great complement to Metallica.

Star trek for sure (or iron man)

ST for sure. Longer ball times to balance out MET, very fast with buttery ramps and deep rules and theme that is enjoyed by anyone 5-75 years old.

ST Premium has a great light show.

Own both MET premium and TWD premium. I would choose TWD premium by a fairly wide margin. I have two young sons as well (a bit older than yours); and they love TWD- it's become their favorite pin. The zombie theme isn't an issue for either or any of their friends. They've both also played ST and find the shots a bit repetitive.
For longer term ownership, TWD is the way to go.

Go play both and then buy what YOU like the most. Isn't it tremendously unlikely your kids, ages 5 and 7, are going to dislike any modern pinball machine you put in front of them?

Avoid the one you will grow tired of in the home environment.

TWD is a great, but tough pin. Lots of gore. Better code than ST. I'd favor ST for your situation.

If you're open to non-NIB, LOTR and SM are excellent.

Most family-friendly and still pretty good pins are Shrek and Pirates.

In that case, if you can afford both I say just get both. With a 3-pin collection, you'll most likely be changing your lineup fairly often. You'll find that even great pins can get stale after a year or two in a small lineup and almost nothing will be permanent. There's really no need to agonize.

In a year from now, or whenever, you can sell or trade the one getting played the least for that Twilight Zone.

Your wife sounds very wise .You are a Lucky Man indeed.

I have MoPLE,TWDLE, and STLE.

You can take away my other 7 pins and leave Me with just these 3