I'm all about the safety of a hard-sided crate for use in a vehicle. But, for an evacuation situation for a couple of little guys like your Chis, I would buy a couple of the soft-sided carriers used as airline carryon pet carriers. The Sherpa or Sturdibag brands are the best made, but you can also get knock-offs at Walmart. They are small, have a shoulder strap for quick getaway, and also have a zippered pouch where you can store an emergency food ration, any meds, some pickup bags, etc.
Do teach your dogs how to go into them without putting up a fight. Once you hear the tornado siren you don't have time to negotiate! Feeding them their meals in there with the door unzipped is a great way to make them like the bag.