I have an Alien mom and her toddler daughter go to eat lunch on a Dine Out restaurant but her daughter can't sit in the table even though I can order food for her. Why this happens? Are EP/GP/SP objects not fully updated for Toddlers? Is there a mod to fix it?

Toddlers weren't designed around add on content.

I wonder if you can add a highchair to a couple of tables in each restaurant and then choose a table with one when you dine out with a toddler. I think the problem is just that toddlers aren't designed to sit at a table, period. They either eat in a highchair, on the floor, or sitting on the couch. But I don't think they are tall enough to eat at the dining room tables.

I took my sims to a restaurant and added a highchair and then all the fun started. My sims kept getting up to check on my toddler and would take her out of the chair and put her back in. I finally had to send her to daycare to finish their meal.

Well..... I guess eating out with a toddler is NOT a good choice - just like in RL! LMAO!

Well, I am planning on getting Dine Out soon, and now I know I'll have to edit the restaurant(s) lot(s) to add high chairs. resto have them in real life, so why not in my game.
BTW, I always had a good experience with my own children. I have 4..and never I regretted bringing them along wherever I went. On the contrary..when I didn't, i felt sad..lol

Does it help putting a fence around the chair, on the sides that doesn't face the table.

It might I haven't tried that.

This is also why I came to the conclusion that they would never add toddlers. Because as they kept adding more and more content, most of it wouldn't be compatible with toddlers. Also I figured that they would have to add them as Base Game content, or else no future packs could ever contain anything for toddlers.

I was shocked when they added them for free. I really did not think they would do it. I guess they knew they had to or the game was just not going to appeal to a lot of people. At least they should be able to design content in future packs with the toddlers in mind, at least if their budget allows it of course. Plus I think they are already working on more content for them. $$$

They will most likely add a toddler pack with clothes and hair soon.

I have that problem outside of restaurants. My sims just don't seem to wanna leave toddlers in their high chairs. Even if autonomy turned off, I have to keep a close eye on my sims. Otherwise family meals turn into the most annoying game of musical chairs....well...even more than than they were before toddlers.