Tomball used to be a small Texas town that has found itself closer and closer to Houston as the city has grown north. It s about as safe as you can get in a city the size of Houston. Look at a map of the north Houston area and concentrate on the places north of Louetta Rd. You are going to find that area relatively safe.
Other than Tomball there s The Woodlands, just north of Houston by a hop, skip jump off IH 45; there s also Cypress, Humble Kingwood.
Here s a link to Tomball s crime statistics.
Most of Tomball is safe enough. Spring is okay. I agree that The Woodlands is the nicest area to the North.