It’s hard to tell but this looks like it “might” be a shaped breastcollar? Maybe it’s not a deep V collar (which would be nicer on this pony) but it seems like it dips a tad in the front, but maybe that’s an illusion, i didnt look at your website to see other photos. So you might be ok there anyway, but if it’s got a ring, i would add the false martingale Drive NJ mentioned, that would help keep the sucker down. If you happen to have an old flash strap off a bridle, that can do the trick and save you buying something else for the time being.
What on earth was the harness maker thinking with they made that breeching? I agree it’s REALLY narrow. A pad would help, but agree that needs tossed for something wider. For a decent looking harness otherwise, it’s funny it has such sad wimpy breeching…
The cart… I honestly would not be using this at all, especially not with a young kid on it. (And i drive with my 2yr old, so i get it! They love it!) But it takes one bounce and she’s over the back of that seat, and with it tipping back already, it’s just an accident waiting to happen. I used to drive with my thigh over one of my son’s legs to help keep him in. But even the most saint of a pony can have a mild spook and jerk the cart… Out goes this kid with nothing there to help keep her in. Something to look at in a future cart purchase, since you do have a young one on the seat, make sure there isnt a big gap between the seat bottom and the seat back. I about lost my son out between the cushions on a hill once! And if the seat is a slick vinyl, get a roll of the rubber cabinet lining stuff and lay a section of it over the seat. Works SUPER! I’m not a fan of lap straps in case you need to evacuate, but i dont want to be sliding all over my seat either.
There is no rule as to what bit is needed or not needed. I personally hate half cheeks because if i needed brakes, i dont have any, and with a kid on the cart, it’s important i know i can stop. My mini drives in a butterfly arch mouth with a curb chain. At home, i drive him with the reins on the top ring, he’s a good boy and i rarely actually NEED that curb chain, but away from home, my reins go on the bottom ring “just in case.”
It was hard for me to find a good assortment of small pony bits, but i found these guys and they are SUPER to work with:
http://www.iowavalleycarriage.com/catalog/29
Just to give you more options.
Kudos to you for wearing helmets! My son just graduated from his bike helmet to a real horsey one like mom’s, so he thinks he’s cool now. 
I wish they made riding helmets smaller. I guess they dont think some of us are crazy enough to put our under 3 kids on a horse/pony/cart… But mine’s been on the cart since he was a year and a half and now he’s on his pony at 2.5, you cant tell him he’s too young!
G & S carts get mixed reviews, they seem to balance out perfect or not at all… But instead of going with a wood wheel, this may be a different option for you:
http://www.gscart.com/atcart.htm
A friend of mine has one that balances out well for her small shetland, her’s has a dash and brakes. Her seat back is wider than the one pictured, but we both complain that the seat bottom needs to be wider by about 4". For kids probably fine, for someone with a longer thigh, it isnt real comfortable sitting in it long term. Hers does ride smoother than my cheaper trail cart. It sure isnt a Pacific Smart Cart, but probably the better of the pipe cart options out there having had my hands on a few.
Mine wears a Country Carriages harness with the super V collar. It’s really nicely padded, but a little pricey. However, now onto my 3rd harness and several hundred dollars down the drain on the junk ones… It would have paid off if i would have gotten the good one first instead of looking for cheaper alternatives!
