They are little, so verbal reminders (especially after they start escalating) won't help. I would suggest the following:
-Make signs that say "Quiet Shhhhh" with a little picture on it. The night before, make it a game and have the kids "stick" the signs everywhere they may be needed... bedrooms, hallway, bathroom, kitchen, wherever.
- Tell them that as a group they get one reminder to keep their voices down. Practice an acceptable volume. You can make a game out of this too (I used to do it with my students)- have them be "too loud, too quiet and then just right". Practice over and over again so they can hear and feel the difference.
- If they need a second reminder, go into "silence" mode. No talking, at all. Don't loose your temper, just come up with a nonverbal signal that lets them know that the talking priviledge is over.
It's a multisensory way of getting your point across. I used this technique with a classroom full of first graders and it worked like a charm for the teacher!