Are there any tricks to make "U brake" stronger? I rebuilt a kids bike. I had the wheelset and calliper/U-brake replaced. Parts that I used are not cheap parts, lets say its more of mid range parts for BMX, normally made of alloy. I made sure the calliper is installed properly and brake shoe are aligned with the rims.

Still the brake is weak, even on a road that is 15 degrees it won't stop. For me it looks like the brake shoe is slipping and not having a good hold on the rims even if its perfectly aligned. I tried pouring soda on the rims to make it sticky but still no good. Any ideas?

Update: I already did try to degrease the rims and used more expensive brake shoe(Baradine brake pads) still no good. Brakes did improve but not what I wanted. I have to give so much pressure on the brake lever in order for it to stop, when I say so much pressure I feel like the brake lever and the cable wire is about to snap.
The easy solution is usually better brake pads. If you're using $2 brake pads, or just old worn ones, you're not going to get very good braking. Spending $10 on a pair of decent pads can make a huge difference.

As well, make sure the brakes are set up properly, especially if you're using two finger (short) levers. With new cables, true wheels and minimum clearance between pads and rim you'll be able to put maximum force on the brakes. 

If everything is properly rigid you shouldn't see anything flex when you apply the brakes. If you do, maybe look at a reinforcing plate (called "brake bridges"). Many older U brakes already come with those. It's a U or C shaped plate than goes over the top of the brake to reinforce the pivots. They're also used with V brakes but shouldn't be necessary if the bike is built properly. Old mountain bikes and BMX's often just had U brake or V brake pivots added to a frame designed for calipers, and the tubes would bend or twist when the brakes were applied. That made the brakes no better than calipers in many cases.