How do I make pants tuck into boots??

So, I have a pair of boots I am converting from adult size down to toddlers and children.  I used the rain boots for the children since they're about the same height.  I opened the rain boots in blender, append the mesh of my boots in, joined the two meshes, deleted the rain boot mesh and saved.  Everything looks good in game, except for the fact that with the rain boots, pants get tucked into the boots but with mine, they dont.  Why would it not be working?  The same thing is happening with the toddlers, I used ugg boots I had created, which somehow I made the jeans tuck in, but when I bring in these boots, it doesn't.



I am no blender expert, so if its something obvious, I'm sorry.
If it is something a bit further along, please go slow. Like I said, still new to blender so try to explain it in layman's terms if you can. That or point me to a tutorial that touches on this. I looked, but couldnt find one myself.
Thanks in advance for any help that you guys can give. I spent all day on this and I am at my wits end.