Hang on though, nobody even knows what happened. Loads of things can smell like hand sanitizer including water bottles which have developed biofilm on the inside, which is not unusual.
A tiny squirt of hand sanitizer in a bottle of water is going to immediately taste unpleasant and not be drunk, but if it was drunk it is unlikely to do any harm. It would be essentially a very weak alcoholic mix.
They are at primary school so if someone had done this, it's probably curiosity and random, rather than targeted malice or a precursor to date rape. 
There is no need for a full scale criminal investigation - talking to the class, and informing the child's parents (given they don't know who did it) is absolutely fine and if someone WAS just doing it curiously, then they know now that it is not acceptable and why.
If this continued and was happening to multiple children, that is the time to investigate more and try to find out who is doing it. But to put a load of teacher time and effort into that, when likely all that is needed is a clear message of saying we know you did this; don't do it again - it's overkill. Teachers need their time and energy to teach, not go around being amateur detectives.