Ooof I don't like to share this because asthma is a serious condition and needs to be medicated. However, we had a really bad time with it.
DC was prescribed it age 6ish. We didn't notice an immediate change, and at that age children constantly change and develop anyway, especially in their personalities. DC grew up to be quite an anxious child, always a bit nervous and scared of many things. It wasn't overly serious- we never needed outside help- but he was always that kid who worries that someone was going to fall or what if the house went on fire, that kind of thing.
Aged 11, we went to the hospital for a check up with a new consultant. Consultant had a chat with DC, in which DC told him about a few worries he had (stuff like second hand smoking, traffic congestion affecting his lungs etc) and the doctor said straight away to stop the Montelukast. He was of the opinion that it was causing anxiety in my DC, and told me that he was seeing it all the time with children and teenagers.
DC came off the tablets, and honestly, within a few months, he was happy, healthy, laid back. A completely different child. He's grown up now, and is still a measured and sensible sort of person, but it really shocked me how we kept him on medication that affected him without really realising it.
Also- his anxiety was a trigger for asthma, so it was ineffective physically too.
I'm not going to tell you not to give it to your child, because treating asthma is so important, but I'd keep a keen eye on it.