New septic tank is going in 55 feet from our well and the installer says that is fine

Horizontal feet is the number the code writes down because it is easy to measure. What actually protects you is the casing depth, the grout seal around the casing, and what the geology does between the two. Ask for the well log, and if the casing is only into sand and not through a confining layer, that matters more than the twenty feet you are arguing about.

Do a baseline test before he digs and keep the paperwork. EPA guidance for private wells is to test annually for total coliform, nitrate, total dissolved solids and pH, so that is your minimum panel, and nitrate is the one I would care about with a toddler in the house. A basic lab panel is not expensive. Then retest a month after the system is commissioned and again at the first spring thaw. If something ever goes wrong you will want a dated clean result from before the work.

Short version: get the well log, get a baseline sample, ask the health department directly rather than the installer, and put the lids where a truck can reach them. Fifty five feet with good casing is not the thing that will bite you.

Politely disagreeing with the whole it meets code framing. Code here is 50 feet to the tank and 100 to the field, one county over it is 100 and 150, and the state minimum is a floor not a recommendation. Call the health department yourself and ask what they will actually permit and whether a variance is being used. If your installer is relying on a variance, you want to know that before it is buried.

We got this wrong. Field went in on the uphill side of the well because that was where the flat ground was, and for two years it was fine. Third spring was wet, we got a coliform positive, shock chlorinated, cleared, got another one in the autumn. Ended up drilling a new well on the far corner of the lot which cost more than the original septic. If there is any way to put the field downgradient of the well, take it.

Worth asking the installer to sleeve the water line where it crosses anything, and to leave you a scaled sketch of what went where before it is backfilled. Photograph the open trench with a tape measure in shot. Fifteen years from now when someone asks where the tank lids are, you will be very pleased with yourself.