I too have kids and experienced the same problem you had using the ARC Pro with the standard two 15V 4A power supplies. Between races and when the ARC Pro was off, the cars were just way too fast.
I went ahead and ordered two 12V 4A laptop power supplies with 6.3mm x 3mm tips (the ends of the stock ARC Pro power supplies are also the same stock ends for Toshiba laptop chargers..apparently).
Anyway, mission accomplished!
It was just enough of a power drop to help the less experienced racers run magnet-free cars like the rest of us.
The power supplies were less than $10 each. Just be careful when shopping for them. Most 12V supplies are going to be only 1-2A. I wanted to keep the amps at a minimum of 4 based on comments from other forums.
Finally, there are a number of laptop adapter tips to convert a laptop supply to 6.3mm x 3mm. I ended up using tip connectors on the two units I bought. The standard Toshiba power supplies were mostly 19V....just the opposite of what I was trying to do.