Sundays can be really crowdy and the last thing I personally like in Erding is to queue up for the water slides, which can take up to 30min on certain ones.
Monday's from 14:00 is family day with reduced entrance fee..so more people..

If I was you I would go in on weekdays and you either buy a whole day ticket or if you go for a 4 hour ticket , walk in at 12:30 and enjoy the wavepool and the other pool areas, even outside pools and then walk into Galaxy at 14:00
have some fun and go back to the other areas when it gets too crowdy.

As mentioned before, if any of the adults decides to go to the sauna area in Vitaloase or Vitaltherme (additional pay) keep in mind..no kids under 16. that's enforced, aswell that all saunas are 100% "textile free" and talking is not that much welcomed within the sauna.

If you go on sundays then as early as possible and then first into Galaxy.

Have fun..
If you have more questions just send me a PM