Hello KAVYOMEE
Mt. Titlis is free for children holding the Swiss Family Card, provided they are accompanied by a parent and the parent holds a 50% off ticket with the Swiss Travel Pass.
As a Swiss living in the Lucerne area, I can tell you, in Switzerland, prebooking a mountain trip that far in advance is not advisable. The weather in Switzerland is constantly changing. Mt. Titlis is an expensive mountain, the weather is changing often, a weather forecast is only possible two days in advance. Currently, Mt. Titlis is covered in big clouds, as I can see from my office. In case of bad weather, with clouds, would you still want to go, and spend the money even if there are no views?
I would advise purchasing the tickets the day before at the SBB Travel Centre in Lucerne train station. A day before, you know the weather forecast, and can still purchase your tickets to beat the queues. The travel agent knows what to sell if you hold the Swiss Travel Pass.
If it is a rainy day, and you do not want to go, the Swiss Museum of transport in Lucerne is a suitable alternative for kids.
Kind regards,
Roland