Maybe do not go to universal studio and spend 2d1n in legoland. Universal studio may not be suitable for ur younger kid. Then 1 whole day in sgp to go to duck tour, sky ride or garden by the bay.
Oh not aware that u have booked the hotel. For USS, guess ur younger kid can't enjoy much the rides. U may save the entry ticket and explore others in Sentosa or even go to SG zoo or night safari and river safari. Bird park is also another choice. Of coz, if baba/mama like the rides in USS then need to keep lah. (emoji)