Most power tools are built to last so as long as they are in good working condition, buy them used.
After a thorough cleaning sporting equipment should be able to hold up to whatever your kids throw at it. Kids grow quickly too so buying new might be a shortlived expense.
No point going cheap when it comes to the safety of your vehicle and its occupants.
Safety standards are constantly changing and it's never wise to take a chance on furniture for kids. Buy it new so you know you're getting the most safe model.
Don't take any chances when it comes to your head!