Hi Liam, In theory, Yamaha grizzly normally operated at 24 volt. It needs two adapters wired serial to run two Makita 18v barriers at the same time. But I have not tested if that is feasible. Some online video shows the speed is way too fast on boosted Yamaha grizzly. It may be not a good idea if kid cannot manage the speed. Instead, I would recommend replacement batteries in below link.
https://durally.net/24v-Highest-Capacity-Rechargeable-SLA-Batteries-Set-for-24v-Yamaha-Grizzly-Monster-Jam-Grave-Digger-Ride-on-Toy_p_20.html