First of all, Swiss pass can be considered as pretty worthy option only in case person intends to do a lot of train travel for middle and long distances. In case of Zurich-Luzern-Interlaken route it is fully useless.
Regular one-way fare Zurich-Luzern is 23 CHF, while Luzern-Interlaken is 30 CHF per person (for additional cost you can add City Transport ticket to your rail ticket). And no need to book anything in advance whatsoever - just purchase your tickets before departure.
For the child take a look at
http://mct.sbb.ch/mct/en/reisemarkt/services/fuer-alle/familie/juniorkarte.htm