You may want to ask in Dental as well, since people who've had amalgams removed would be in a similar position--there's a burst of mercury with both the placing and the removal of amalgams as well as the daily leaching.
I just had my amalgams removed 2 weeks ago, and I can share what my healthcare provider recommended. As background: my health has been bad for 2.5 yrs now because of the mercury in my fillings and my kids are affected as well (my younger more than my older), and our healthcare provider is someone with a background in environmental health and specifically mercury.
Except on the day of the removal (don’t take vitC then because it will make the local anesthetic wear off really fast), I need to take lots of vitamin C, to bowel tolerance. That can be a lot. When I started supplements for my kids, my son (about 30 lbs) needed 9 grams per day, and my daughter (40 lbs, and my "healthy" kid) needed about 30 grams. This is highly individual and your child may be vastly different, but I included my kids simply as reference points. I divide it into 3 doses spread out over the course of the day. I use sodium ascorbate because it's gentler on the stomach and doesn't have the harsh, acid taste of an ascorbic acid—it just has a salty flavor that mixes well in liquids (not hot). Whole Foods usually carries sodium ascorbate, or you can order from Bronson, item 50B.
I was told to take extra DHA, I don't know if you've got a DHA supp now, there are lots of brands, or you could use something like Carlson's cod liver oil which is high in DHA/EPA and give about a teaspoon 3x/day. You won't overdose on A or D at that level (not nearly).
A good multi will also help with the vitamins and minerals necessary for detoxification. My HCP recommended Rainbow Light as the best of the brands that are widely-available, and in this case, I think taking more than the amount on the label is fine—the idea would be to provide lots of nutrients to help support the body. I’m giving a small raft of other supps to the kids to help with detoxification as well—it helps with stuff on a daily basis (new toxins) as well as catch-up for all the accumulated stuff they got from me. I think it’s the basic biomedical-type supps, if you want to know what we use, PM me.
On another note, it sounds like you're doing all the things you need to do. I also would be incredibly angry, but it sounds like you're using that productively to take all the steps that you need to, so good job to you!
ETA: Some of the basic stuff for reducing the amount of mercury that leaches out: no chewing gum, not sipping on really hot liquids (more for adults who drink coffee/tea, kids usually eat/drink things that are warm-ish and much less an issue), avoid cilantro, especially raw, and if your child grinds their teeth, wear a nightguard (I think some basic ones are even available at drugstores, they don't need professional fitting). There are probably more, but this is all that comes to mind right now. And again, it sounds like you're doing a really good job dealing with a really difficult situation.