This is a huge trip. I did it with my children about 4 weeks ago, and I honestly do not recommend it.
We were collected from our Sentosa hotel at 8am. Taken to the bus depot. Left on the bus at 9.15. You get off the bus at Singapore customs.... then you get off yet again at Malaysian customs... filling out forms, processing passports blah blah... Finally about 1.5 hrs later you arrive... BUT the biggest pain of it all was getting back
You have to again fill out forms at both customs, get all of your personal belongings x-rayed. So its about 6pm when you finally get back into Singapore to.........PEAK HOUR traffic. We arrived back at our hotel absolutely exhausted and shattered at 8PM!!!! Never again! almost 7 years ago