Trailgators are great. Make sure you do everything up nice and tight, and take a spanner with you on your first trip as you won't have tightened it enough. You need to be confident that your nipper won't brake and / or try to get off at inopportune moments – not fun trying to safely cross a road only to suddenly slow down half way across because Junior doesn;t like going so fast!
Take note of your kid's feelings, some are fine and others will get scared at what you consider to be a slow speed.
When you are riding solo, the towbar does make the back end a little noisy and unweildy, but no great shakes if you're just cruising – just don't go jumping!
Oh yes, the bracket on the seatpost is supposed to be a permanent fitting, so make sure it won't affect other use of the bike