I think if I were you, I'd go with the K math that's planned in LHFHG. It is pretty basic, and may not seem incredibly challenging every day, but it really lays such a solid foundation. When Garret (my now 6 year old) was 3, he tagged along with his older sister in LHFHG during our days. He is very math-minded and loved the activities in LHFHG. He didn't use the workbook that year, but I know that he understood each and every concept. He just turned 6 this week, and has used LHFHG for his K year, and we have been doing the math with LHFHG (even though he already understood all of it 2 years ago). Having seen the progression in Singapore with his older siblings, I know that the concepts will get more challenging quickly enough. He is enjoying it and thinking, and I'm happy with that.
I know that I probably would have been tempted to think it was too easy or that I needed to push more if he were my 1st though.

I have gained some wisdom from others...and my own mistakes...along the way.

I know that for K, there are many new things that kids are learning to do. Converting sounds to letters in writing and reading, writing down thoughts (in storytime weekly), writing numbers, fine motor skills, gross motor skills, listening, narrating, etc. Each and every box in the guide is not meant to be a challenge every day. Some new things will be challenging, and that will vary from child to child as to what's more difficult. Other boxes will still require thought, but not be the most challenging thing in the day. So, it's ok of the math makes her think, but isn't a big challenge this year. It will help give her a solid foundation for the next year. Now, I'm not saying that you can't use the 1A, 1B books, but just thought I'd share the perspective I've gained a ways down the track with my kids.

We love HOD! And, my math-loving little guy is talking about figuring things out all the time in real life. He's solving problems, dividing things, seeing patterns...

Kathleen