I asked Carlton city about 2 adults and 2 kids before (6 and 4 years old), they said their twin bed is super single sized, so I think at most 3.5 feet wide.
Eventually I stayed at swissotel merchant court with two 4 feet beds and can request complimentary bed for the kid. It is just above Clarke quay mrt. The price is similar as far as I remember .
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From Carlton city :
The rooms can occupy a maximum of 3 persons. As your children are still of a small age, you may select a king sized bed and have an extra single bed for the room at an additional charge of $75++ per night.
The twin beds are super single sized for 1 person’s comfort depending on individuals.
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