awkward pay debate, what's my next move?

I have been with my family for just about 2 years, and it has been absolutely great. Its 3 girls - 7, 4, and 7 months. I would say for the first 6 months I got paid in cash and the rate varied between $13-15 depending on increments of 20 (the dad would just grab cash from an ATM) and I didn't mind because it was generally reliable - I knew my total at the end of the week. He even corrected his wife a few times when she didn't know my rate and he always based any checks on 15/hr. This past summer they started paying me in weekly checks that were based on $15 / hour, both before and after the baby was born. Recently the father (who is stay-at-home and who is usually the one I talk to and see often) has tried to underpay me and then made comments, acting shocked, about how "generous" it was that his wife paid me 15, especially since the older girl was at an afterschool club and this supposedly makes less work for me. It just made me extremely uncomfortable that after this long, suddenly my pay is under debate and that he said that to me, rather than talking to his wife. Inappropriate. Yesterday he wrote a check - for the wrong amount - and put it next to my bag before he left, rather than handing it to me. I found it when he left with the kids and I was packing up to leave. In my opinion, once an hourly rate is established, that's it, you don't lower employees' pay. My pay has never before depended upon the number of kids happened to be home during my schedule. I am going to approach him about the incorrect check, with the wife/mom also present, but I am nervous because he already tried arguing with me about it that other time - What should I do if they want to change my pay system? Nothing's in writing, I'm not registered anywhere - I feel very vulnerable.

I am making 12.00 an hour for 2 children. I think you well earn 15.00 an hour for 3 children. If things continue this way I would start looking for new opportunities. I am in California but I use a website care.com and sittercity.

Hi K, I think it's very unfair what he does to you , and I would get a new job and just walk out the door one day.