It's important that you don't make them feel bad about not going in the toilet. I'm sure it's not that they're trying to defy you, but that they just have trouble getting their bowels to cooperate. They may just not be ready to poop in the toilet yet.
My daughter is 2.5, and I thought she was ready to potty train, but when we put her in big girl underwear, she kept going in them, despite being uncomfortable. She's just not quite ready to go in the toilet yet, so we're going to hold off a few weeks.
I know you're frustrated, but try to reinforce the positive, not the negative. Give them lots and lots of praise for pottying in the toilet, and give them incentives to poo there as well (candy, small toy, etc.). I remember my sister told her daughter that when she pooped in the toilet, she could go to TRU and pick a special toy. But try not to tell your kids that they're babies or punish them in any way, because I'm sure it's not a pleasant experience for them, either.
Also, many kids get constipated, especially when potty training. If your children seem to strain when going or if their poo seems hard, you may ask your pediatrician for a gentle laxative (like Miralax) to help them go. It may be hurting them to go, and the toilet is scaring them.