I say, "GO FOR IT."
Our children are five and three and they have been to Disneyland three times already. Each time was a completely different experience, but our best was the last trip we took.
We spent three days at Disneyland and the third day we got to go in before opening. It was not that costly as my daughter was still under the paying age. We enjoyed having the luxury of leaving the park to go back to the hotel to nap and rest. We didn't feel like we had to 'hurry' the kids through the parks. If there was a long line, we waited until we went the next day. I also HIGHLY recommend a character breakfast and since it will be your daughter's birthday, one with the princesses would be fabulous!
My only recommendation would be to take an extra family member. I know that you and your husband can switch off taking the older child on rides, but it was nice for my husband and I to go on some of the rides solo. The first time we went we took a family friend and the second and third time grandma and grandpa came along. Just a suggestion, but not a necessity.
Have fun. I have a great recommendation for a hotel let me know if you would like the name.