Very nice spa and waterpark with three areas including large sauna area (which means over 16 and without clothes only!). There is enough to do for a 14 year old, particularily the water slide park.

The HIX is very remote in an industrial park, and I would suggest you stay in downtown Erding which has a great old town area. For example at the Hotel Erdinger Weißbräu, and take a taxi to and from the Therme.