My husband the tax accountant told me to talk about tax deductions.
The seats are fully deductible if your ckk only ride in them, not your kids. If your kids ever ride in the seats (more than, say, in an emergency), you should only deduct the percentage of the price that's the percentage of the time they are used by other kids. (For example, if your kid rides in the seat 3 out of 10 times and CKK rides in the seat 7 out of 10 times you may deduct 70% of the seat's cost.) You can also deduct your insurance increase or the cost of your separate liability insurance as an expense. You should take these as business expenses, not itemized deductions, and you need to keep the reciept (if bought online, keep the invoice showing that you paid and when and to whom), not rely on credit card statements. You should keep them for 5 to 7 years.
If you have additional tether anchors installed in your car in order to tether your seats you should not deduct that expense even if your own kids don't ride in carseats and the anchors are used only for your CKK. My husband does not think that is an allowable expense or that a good-faith argument could be made that the IRS would accept.