You shouldn't be messing with them. Liability issues aside, it means that the bike mechanic (apologies to any real bike mechanics reading this) isn't learning what they are doing wrong. Write to the store manager and send copies to Walmart head office, local business standards office and a range of local media outlets as well as anyone else that Walmart may be particularly embaressed to have find out about their incompetence. Focus on the danger to kids and you could well inspire a valuable mix of outrage and action.