letter to raise rates for only one client

[WImom]

I have a min. of 3 days for all clients. I have one client that only comes two. I made a mistake when I first started letting them do this (2 years ago) and I don't want to lose them so I'll leave it be for this child's last year.
I charge them $80 for two days. I just noticed when I raised my rates for new clients this last summer to 3 days at $130.00 these three day clients are paying more per day then my 2 day client. I'd like to charge $90 per week for this 2 day client. How should I word this?

[Mom&Provider]

Dear DCM/DCD,

As of (date), my childcare charges will be increased to $90 per week for P/T care of two days per week. It has been X year(s) since I have last set my rates, and this new rate will enable me to continue providing the best of care for your child.

Would you kindly initial and date one copy of this letter, and return it to me for my records by (date).

Sincerely,
Provider


ETA: I'd be prepared to have some reasons for the increase or include them briefly in your letter, even if it's simply cost of living and it's valid to you to keep them at 2 days per week since you don't offer this anymore - which is hard to justify since you accepted those terms from the start. I'd also mention (in person not the letter) that they have been enjoying a cheaper rate for some time now compared to other parents and you are wanting to bring everyone up to your current rates. Its over $500 extra per year, so it depends on how the family looks at the cost increase....