Why can't I bring toddlers along to a Sims birthday party?

It's strange to me, that my Simself with family of husband and two toddlers. Took in Wolfgang along with Gunther as I matched Gunther with my Sim's close friend and was neighbor, Liberty. To have someone close to grow with the last baby of Simself. It makes sense to bring the toddlers to be at Wolfgang's birthday party and any other Sim's birthday, but their not listed to bring. Gunther is at work, but I can get Lucas and Mila.