Definitely a cute idea and I'm a sucker for drafts.
I'm going to gloss over the horses as your other critiquers mentioned some things on them, but I would point out that young horses, especially young drafts, tend to have large, knobby joints. Their knees and hocks tend to be accentuated and their legs often appear to be almost as long as a full-grown horse's from the ground to the belly. Your foal has almost adult proportions with the wither to belly being about the same as belly to fetlock. A more foal-like proportion would put the leg length about double the dept of the body.
I am going to focus on the tack. Harnesses are not the easiest thing to draw, especially if you haven't been around them in person as there are a lot of things going on in a harness that you can't always glean from a photograph or 400x600 pixel image off of the internet. I would recommend my own tutorials on harnesses, though I do admit this just covers two particular harnesses and does not constitute a complete, comprehensive guide to all harnesses everywhere. [link]
The two most important components in any harness are the collar and hames (in this case, the other style is called a breastcollar harness) and the traces/tugs. The collar is the large, ovalish piece that sits around the horses neck, it basically serves as padding for the hames, which are often metal either black or stainless steel (fancier ones tend to be stainless and you do occasionally find wooden hames, but that is more in Europe than the US where the Budweiser horses reside). Attached to the hames are the tugs/traces, which then essentially attach to the cart, carriage or wagon (more details are there, but I'm trying not to be too verbose!). The traces are essential to any harness, they attach the horse to the carriage and that in turn allows the horse to pull it. Your left lead horse is lacking in traces and is technically not attached, but standing there of his own free will and choice. Yes there are choices to be made when stylizing, like not including the breeching on your lead pair, which often times a lead pair/team does not have a breeching because they're not responsible for backing up the vehicle, but all of the horses will have traces.
Most horses are driven in blinders, but I know for the purpose of seeing the expressions on the horses' faces they were omitted. The reins should fall more to the side of the collar rather than going through the ring you have at the top of it (this space is actually decorative if you're simplifying the scotch collars an they often have a little ornament hanging from it).
It really is a cute piece and an adorable idea, one I hope you might revisit sometime!