how do we make sure we aren't liable for any accidents?

I think the reason why you do not see toddler bunk beds is because they are too inherently dangerous to sell.  I cannot imagine a parent in the world who would want to put a three year old in a bed five feet off the ground.  However, assuming that you are able to sell the beds given the abundance of parents that lack common sense, I would also imagine that the product would be a huge target for products liability, personal injury, and perhaps wrongful death lawsuits.  Therefore, even if you structured your company to sell the product through a limited liability company to insulate yourself personally from liability and added the appropriate disclaimers/warnings to the product, I think that it would be very difficult for you to obtain insurance at a cost that would allow you to price this product competitively.

In my opinion, there are too many potential liability issues with toddler-sized bunk beds.  Toddlers are notoriously accident-prone due to a lack of developmental maturity.  They fall easily and do not have the maturity to make good decisions when it comes to taking risks.  A bed for a toddler should be low to the ground and have a railing.  At least if they get over the railing, they don't have far to fall.  With a bunk bed, it is too potentially dangerous.  You cannot protect yourself from that kind of liability as a business.  I hate to say it, but I think it is just a bad idea.