Let me answer this two ways:
(1) Hmm, good luck with the hope that they differentiate in 1st grade - especially if you now help your DD to get ahead! Unless you're very, very lucky you'll find it easy, with an enthusiastically mathy child, to get way outside the range of the differentiation they're equipped to do. You might want to consider using logic puzzle books rather than maths syllabus books - e.g.
these Mindware ones (my DS enjoyed some of the codebreaker ones and the perplexors) or
these Tarquin ones (most will probably be for later, though). If you get these at the right level, they'll encourage mathematical thinking and be fun without explicitly doing stuff that school will teach later.
2) DS's frustration with doing nothing more than counting in his first year at school was what drove us to ALEKS. Their first course is third grade, but since prerequisites are included in the course, you absolutely don't have to have done all of first and second grade material to get started; you can do it in the course. These questions are not logic puzzles: they are very straightforward applications of basic techniques. However, my DS enjoyed it a lot at that stage; I'd say it was a good alternative to school-style workbooks, as it has the same kind of questions but more intelligently and flexibly presented (can test out of what's already known). The downside is that a keen child can go through years' worth of maths in a very short time and then you really do have a differentiation problem...!
