One of the exciting milestones of parenting is when your child is ready for his or her first big kid bed. But how do you know when it's time and what is the best type of bed to buy? The options can be dizzying -- bunk bed, trundle bed, bed with rails, twin bed and so on.

Before you embark on the shopping trip, consider the child's age and social readiness for the big move.

Most kids are ready to graduate from a crib to a toddler bed between ages two and three, says Dr. Steve Abelowitz, the medical director for the Coastal Kids Pediatric Medical Group in Newport Beach, California. "Some of the signs to look for are if the child starts wanting to climb out of the crib," he says.

If you've got yourself a little monkey, then safety is a big concern. He or she could hurt himself by finding creative ways to launch himself out of the confines of his crib.

Your child might also ask to sleep in a big kid bed. Or, if you're expecting a new baby, you may want to move your toddler into a bed so the new arrival can use the crib.

Many parents move their kids from cribs to beds in stages. For instance, you can opt first for a low toddler bed and then move to a real bed later, advises Dr. Kate Cronan, the medical editor for Kidshealth.org.

She recommends choosing a bed with guard rails for safety. Remove them as a child grows, and then place a carpet or something else that is soft on the floor in case your child does fall out.

The trundle option works well for the occasions when your kid has a friend come and stay overnight. Many child beds come with drawers underneath, which are a nice feature if you need added storage. Kids also love bunk beds, but do make sure your child is ready for one and won't roll out of the top bunk.

Good places to shop include Target, Sears, Toys "R" Us or a locally-owned furniture store in your neighborhood.

For your own sanity, do buy a waterproof mattress cover. When your child moves out of pull-ups at night, you'll be grateful for the investment.