There are some great serviced apartment options available in Singapore. These are especially good for 7 night stays or longer as one of the taxes doesn't apply (at least that used to be the case).

The one we used with our kids on a few occasions (first time was just with one of them under 12 months) was Lanson Place opposite the old post office on Penang Road. We would send management a list and they'd fill the fridge with groceries then just charge us cost price - it worked very very well. Would end up with a high spec 2BR apartment for the same price as a hotel room!

The old Mandarin on Orchard Road also used to have a great deal on connecting rooms, but I think that's being refurbished. Often you have to hunt around each hotel's website to find this stuff...

Did three 1-week trips to Lanson Place but the Shangri-La is where we stayed on our last visit 7 years ago as we were only there for 3 nights. The Valley wing was great but the property is out of the way. Our youngest was 5 at the time and walking one way down to the top of Orchard Road was about as much as we could expect. I guess it's a bit better now with the new MRT station but otherwise you're relying on taxis and there was often a shortage (or the hotel's infrequent shuttle), so just be mindful of that too. As nice as the SL is, you might be better off looking for somewhere closer to the river or Marina Bay. That whole Gardens by the Bay was an absolute hit with my kids on that last visit.

The Paradox on Clarke Quay certainly used to have family rooms/connecting rooms when it was a Swissotel but I can't get the website to load at the moment.

And finally I don't think I would rush to stay on Sentosa. Sure there's lots to do there but you could break up the city time with a couple of day trips down there, most critically meaning you get access to all the great food in the City rather than what I recall being very generic "theme park style fare" on Sentosa.

Very envious as just last night the kids were asking when we could go back!