First line of defense: The flu shot, for everyone, every year. Listen to Christy Lee, below -- she's an EMT so she should know. Even if you just had it, get your own shot as soon as you are able - the flu season this year will be a long and vicious one and you could get it again. Your husband and child need them too. Children are especially vulnerable to the most dangerous complications of flu. (And don't listen to anyone who says it "gives you the flu" etc. -- that is a total myth. Listen to your doctor instead.) You can still get the flu with a flu shot but the vaccine means the illness is much less severe if you do get it! I know that from experience.
Otherwise:
Tons of handwashing. Keep hand sanitizer gel in the cars and in your purse and use liberally. Remember to wash or sanitize after grocery shopping, any other shopping, visiting your child's preschool, etc. Be sure your child washes her hands just before leaving preschool or leaving a play date.
When anyone is sick or even seems sick, I wipe down doorknobs and sink handles and spigots with Clorox wipes and if someone's definitely sick, I do it plenty more. (I just had a bad cold but the others didn't get it because of the wiping down and the next thing.....)
Isolate a sick person. When either my husband or I is sick, the sick one gets our bed and the well one sleeps on the couch. I think it really helps prevent couples from sharing illnesses. I also have the sick person use only one bathroom while the well folks use another. I also spray down the sick person's bathroom with Lysol daily. (If it's gastrointestinal illness, there is even more cleaning that goes on.) I change the sheets and pillowcases before we share a bed again, and I am glad to throw out pillows frequently and replace them -- yep, I buy new bed pillows if someone's been sick. You know those bed pillows are going to hold germs and viruses even with pillowcases on them.
The isolation also means no hugs or kisses for a few days, which my daughter just hates, but she's older than yours and able to understand the point!
Like someone said below -- do not share toothpastes. I throw out my toothbrush if I've been sick or I disinfect it.
Eating fruits and veggies helps over the long, long term but in the short term the best things are a ton of handwashing, frequent use of hand gel after being out in the world, and really just avoiding crowds -- I just don't go to the mall at all unless I get there in the middle of a weekday, for instance. I was in the mall on a weekend recently and it was heaving with crowds and tons of sneezing people and coughing kids -- I fled pretty quickly. Not worth it!