Can singulair (montelukast) for kids work straight away?

Yes: Singulair or montelukast starts to work within one hour of taking a dose. However, for asthma it will start to work quickly but it will not reach the maximum anti inflammatory effect for 2-3 weeks.

Pls see answer: Singulair (montelukast) is not a quick acting medication. If your child has asthma and has symptoms of asthma, typically albuterol inhaler with a spacer device is used to quickly relieve symptoms. Singulair (montelukast) may take days to weeks to be effective.