Singapore Math is typically considered to be a year advanced, so your ds would be placed quite typically by doing 1A/1B for 2nd grade. We did Singapore from K through 6th, and went from Singapore 6B right into Foerster's Algebra 1, so you may find your dc are not 'behind' at all!

We've loved Singapore Math. Here are a few threads about it...
Singapore Math (why Carrie picked it and my top reasons for using it)
viewtopic.php?f=6&t=4271&p=31537#p31537
viewtopic.php?f=10&t=112
However, any math can be used with HOD. It is just nice to know why HOD chose Singapore as you are making your decision!
In Christ,
Julie