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The MGP VX4 Team scooter comes with oversized bars (see MGP page showing that it has HIC, which means oversized bars), so replacement oversized bars would work! :)
Try heating up your bars and HIC shim with a hair dryer or heat gun. Be VERY CAREFUL, but it might make the metal expand just enough to releive some pressure and allow you to pull the shim out.
The only difference between the Ultra Pro and the Ultra Pro Lo is the height of the bars. At 8 years old, the Lo is probably closer to the right height. By the time he's ready for taller bars, he'll probably want a new scooter or replacement bars.