I had a similar problem and bought this and this for a 12V power wheel. Battery has its own BMS to protect against a variety of conditions that power tool batteries don’t have onboard (because the BMS is in the tool). There seem to be quite a few 6V LiFePO4 options. You’ll just want to look for a higher AH rating than you have now. You’ll also want to make sure it has an onboard BMS. The BMS will also limit the max discharge current which I calculated to be about 15-17A for our 12V Jeep. The battery I shared has a max discharge current of 20A.