What can I do to pedals to help kids or smaller riders use the bike

There are always options.

_{From OP's link, showing boom and bolts.}

0. Shorten the boom, if you can. There are two pinch bolts in the main frame, under the boom Back them both off and see how much can be inserted into the frame. You probably already did this, but revisit it and see if there's extra length to be saved. Perhaps the inner tube bottoms out, and you can get some distance back by trimming the end that is inside the frame.

   Downside Its really hard to extend again in the future, so be miserly at cutting. Or try the other options before whipping out the grinder.

   
   ^{from https://tadpolerider2.wordpress.com/tag/tadpole-trike-boom-adjustment/}

1. Block the pedals with wood. This will get you closer, but the full distance would need 4 inches / 100mm of wood on each side. Example:
   

2. Shuffle the seat forward even more. Hard to be detailed without knowing how your seat is mounted. The backrest can often be adjusted for tilt but that doesn't move the hips forward which is required. You might find that reclining the back counterintuitively helps push the hips forward.

3. Thicker cushion - Adding a "tailbone cushion" between the hard and soft layers of seat might buy you another half-inch or 12mm perhaps.

4. Crank shorteners - the cranks are not the problem, its the distance to your bottom bracket (front bracket?) and therefore the pedal the farthest away from the rider. There exist bolt-on crank shorteners which will move the pedals closer to the BB axle line, effectively bringing the frontmost pedal back a bit.

   Shorter cranks are common on recumbents.

   

   1. You can also drill and tap cranks to make them shorter permanently, though this is risky and requires some excellent machine shop skills to get the hole angle right, and the threads tapped in straight. My adult sized `bent has 160 mm cranks, which were professionally shortened by a previous owner and you have to look hard to tell. Depends on having enough metal in the right place too.

      Most pedals are 9/16" thread with 20tpi which is weird (18tpi is common but incompatible) and only common on bikes, and the left pedal is left-hand thread, ie backward. So its not a home-shop job.

      Given this is a kids bike, it may also have 1/2" pedal threads, though your specs link doesn't say.

      

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      More info at https://bicycles.stackexchange.com/a/64370/19705

You don't want to use swinging crank adjusters like the Pulse Swing Cranks; they will do the opposite of what you want.