I have no idea if you are using the few drops of rum in your child's hot toddy. If you are I'd only give the hot toddy a spoonful at a time, as needed. When the cough is heavy. If you are only using the honey and lemon you can give your child a small cup to sip while it's still warm (not hot!)
My favorite way to give the hot toddy to my children at that age was to add a couple of tablespoons to a cup of warm Chamomile tea. This would not only calm the cough but it would also calm the child and help promote their much needed rest. If you don't have Chamomile tea on hand you can add it to just plain tea. This keeps them sipping on the hot toddy without drinking it down too fast. I know my children loved the taste of a hot toddy so much they would gulp it down and they need to sip on it. But what I love about it is you can rest assured they won't over-dose on it and have side effects.