I'm doing almost the exact same thing in the next couple weeks to a F150 Power Wheels. I'm going to be using 2 18v batteries in parallel. I decided to be safe and add a fuse to the positive of each battery (30a). I'm combining the two battery leads into a distro block, and from there into a 40a breaker switch so to prevent damage to any of the components down the line should the system draw too much from both batteries at once.
40a Breaker
https://www.amazon.com/gp/product/B000K2K6YS/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&psc=1
From the 40a breaker, I head out to two places just like yours.
One lead heads to the step down to run lights on a dash switch. Again, I added a 10amp fuse inline on the lights to prevent any weirdness there.
The other lead heads to a voltage regulator that will then head into the motor wiring harness (shifter, pedal and then motors).
After looking at your diagram, I'm not seeing anything that looks off. I think it will turn out great! I would recommend adding an inline resistor on the brake pedal (orange wire) to limit the instant off on the motors to help prevent the wear on the motors. I choose
https://www.amazon.com/gp/product/B008MLL3JQ/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&psc=1
I'm also adding a volt meter on the dash to let the kiddos keep an eye on their battery life. The battery controller chip (same as yours) will cut off at 16.5, but I figured they'd at least know when it was time to head home when it hits the high 16's.
Other parts
https://www.amazon.com/gp/product/B07929Y5SZ/ref=ppx_yo_dt_b_asin_title_o06_s00?ie=UTF8&psc=1
https://www.amazon.com/gp/product/B071NQ5G71/ref=ppx_yo_dt_b_asin_title_o06_s01?ie=UTF8&psc=1
https://www.amazon.com/gp/product/B01EFUHFDU/ref=ppx_yo_dt_b_asin_title_o06_s01?ie=UTF8&psc=1
https://www.amazon.com/gp/product/B07TYSP7KV/ref=ppx_yo_dt_b_asin_title_o06_s00?ie=UTF8&psc=1
Not with dewalt. You need a low voltage cutout chip otherwise you will fry the batteries. Other manufacturers have the low voltage cutout built into the battery, dewalt has it built into their tools. You will either need to add that extra chip, or pick a different battery brand.
Example chip.
Digital Low Voltage Protector Disconnect Switch Cut Off 12V Over-Discharge Protection Module for 12-36V Lead Acid Lithium Battery https://www.amazon.com/dp/B07929Y5SZ/ref=cm_sw_r_awdo_NM5NWGSS45V8EQ02AN7M
These work great
IS Icstation Digital Low Voltage Protector Disconnect Switch Over Discharge Protection Module for 12-36V Lead Acid Lithium Battery https://www.amazon.com/dp/B07929Y5SZ/ref=cm_sw_r_cp_api_i_G-LCFb36VDD0G, depending on the battery, look at specs to see what is considered the minimum.
I’m using this for a cut off. https://www.amazon.com/Digital-Battery-Low-Voltage-Protection/dp/B07929Y5SZ
Looks good except for one issue: I assume the LVC you're using is this one : https://www.amazon.com/Digital-Battery-Low-Voltage-Protection/dp/B07929Y5SZ/ref=sr_1_3?dchild=1&keywords=low+voltage+cut+off+12v+30a&qid=1630413548&sr=8-3 LVC is only rated to handle 20a. These have a tendency to go bad as a PW can pull 30a form a stand still. My suggestion is to have the LVC be a trigger for a relay that can handle 30-40a.
Higher RPM gearboxes do lose low end torque. 15k rpm is a good middle ground and what most use to achieve 5mph.
It'll be obvious the remote functionality will be unnecessary after a few solid months of riding. My 3 year old required all of a week worth.
if you're using SLA in series make sure to have matching AH ratings so you aren't discharging one battery more than the other. I recommend spade connectors for attachment to the battery.
Also consider you will need to be able to charge these batteries. I use a 4 flat trailer connector that's tapped into the battery terminal connections. I then use 2 charges 1 that is set for 12v and one set to 6v. When I plug in the 4 flat each charger is connected to the individual battery.
Fuse - Just spice it in with spade connectors.
Most wiring i recommend 12gauge size
LVC's are available on amazon use them as a trigger and not a load handler. Example - https://www.amazon.com/Digital-Battery-Low-Voltage-Protection/dp/B07929Y5SZ/ref=sr_1_2_sspa?dchild=1&keywords=low+voltage+cutoff&qid=1628005038&sr=8-2-spons&psc=1&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUExSTgwMEtMTVpQUUlSJmVuY3J5cHRlZElkPUEwNDkwMTUwMjIwR0hGVlQ4TFBZSyZlbmNyeXB0ZWRBZElkPUEwMDY1MTczVkJWWEcyN04ySTdSJndpZGdldE5hbWU9c3BfYXRmJmFjdGlvbj1jbGlja1JlZGlyZWN0JmRvTm90TG9nQ2xpY2s9dHJ1ZQ==
Best practice is to use an LVC that signals a Relay (Relays handles the load).
Ryobi 18v batteries are a good option as they have an LVC built into them so that eliminates the requirement. I don't use drill batteries in my PW's so I can't comment on them too much.
Going 24v board isn't a bad idea to retain the functionality and i believe some operate in the 18-20v range. Only issue is the remote steering motor is also receiving the increased voltage (Causing it to turn super fast) I'd recommend a buck converter to step the voltage down.
When buying batteries measure the dimensions so you can make sure they fit.