We used the Singapore My Pals Are Here series when DS was that age.... It would be a good candidate for official curriculum submission because other than being a good series (although they've changed editions since, I'm assuming it's just as good!) the workload is very light... so it's easy to get through without stress. Excellent critical thinking, but not a ton of work, so there's plenty of time left for you to add in whatever extra topics or reading you would like.

Erica