What wheels do I need for a cargo bike?
 

Building a cargo bike, front load with two wheels on the front. I would like to use smaller wheels, as it gives more room to use in the cargo area, rather than having two large wheels sticking to far up. I plan on using 20 inch wheels on the cargo area.

Main question, what is stronger? Getting two kids bikes wheels, or using bike trailer wheels?

I want to make sure that the wheels will be able to hold and last in the cargo area, up to 200 lbs. I feel like kids wheels are better, since trailer wheels are generally thinner and meant for lighter loads.

Neither.

What you need is double walled 20" BMX wheels with 36 spokes and 14mm (9/16ths) axles.
This will give you all the strength you need along with a wider selection of tyres to choose from.

Alternatively you can follow my lead and use 36 hole hubs with 14mm axles and have them built with Moped rims, which is a much stronger and better option all round in my opinion and is what I use when building cargo bikes and delta mule trikes, For the tadpole trikes and quads I use 36 hole 1/2" stub axle Sturmey Archer drum brake hubs with moped rims.

Okay, but I can not be spending hundreds on just two front wheels. I do want support and strength.
So would getting BMX tires be a better option? I can find them online for a decent print per wheel.

You can buy alloy double walled 36 spoke 14mm axle BMX front wheels ready made from about $45.00 apiece from ebay and amazon.

However, if you are supporting the axle on both sides of the wheel as many of the these commercially made mule delta trikes are, then a 3/8th axle will suffice, and these can be purchased for around $30.00 each.

Don't skimp or penny-pinch on the wheels …………. they are what makes a cargo bike happen!

Me, in Canada, it is hundreds. While I can find the wheels on ebay, they are much more pricey, or does not ship to Canada.
Buying parts in Canada is a huge pain.

Must fabricate a steering system using 2 forks,
or you have to budget for single side mounted hubs and build font end around those.

Christiania trikes, build box and wheels as a unit with handle bar on the back of the box ,

steering, head tube angle, near vertical of course..

Yes, the two fork style is what I am planning on building into it. Much more strength than just the regular bike handlebar with the one pole. The design I found is from an another bicycle company that makes their own cargo style bike. Theirs is built like a tank, huge tires and frame, it looks awesome. But way to much for what I need, haha. But the style of the handlebar and frame for it is much better, so will be making something similar.
That single side mount is very interesting, and would look a lot cleaner. I will add this to my list of options, thank you :)