It all looks good at first glance - however, east is east and west is west - that's where the rub will lie. 5 years is a long time to be away in a different culture - and it is very different culture in many ways
As far as that $1400/mo - there are many "details" that wil deduct from that amount. Count on about $12k - $13k (sing dollars) max/yr for a $30k school bill for EACH child at most any private school. $3900/mo won't get you much either if you wish to be in the more popular expat areas - more like $5k - $7k/mo.
Don't come to Singapore all starry-eyed or you will regret it. There are many, many positives - but the few (large) negatives can ruin it all over an extended period of time. There is a reason there are so few Americans left at SQ and it is mainly due to those negatives that weigh more and more as time passes. These issues, as mentioned, are mostly cultural issues and they can and will eventually undermine everything if you let them.