The suspension systems are nice but I think a little overkill unless you plan on having the trailer for years. The kids will get jostled a little but it is a good thing for the most part, kind of soothing. There probably will be some bigger bumps but those would be more the result of your going in an area you do not intend to go.
If you are really concerned and do not have suspension, you can do cheap suspension. Buy the fattest tires that will fit your rims and underinflate them. Don't underinflate too much though as you will risk more flats. Say pump a 60 psi tire to 40 or 45psi.