Lots of people put the mattress on the ground when the LO start to scoot and crawl. We did it with DD1 and it worked out really well. She was able to learn how to safely get off the bed pretty quickly and never fell off the bed (although, she did during a diaper change before we put the bed on the ground... ouch). We have had each of our children in our bed from birth (still have the little 2), and have not had the bed on the ground again, but LO is always between DH and I and we never have a problem. I never have DS nap on the high bed though, he is always on a lower napping surface (or mostly on me...).
SAHM to Chloe«- 6/2008 (10 lbs, 5 oz), Hannah- 9/2010 (9 lbs, 12 oz), Liam- 2/2013 (9 lbs, 6 oz)