Yeah engineer dears guide me that what is the minimum ac voltage will give shock to child woods.?

15 volts is quite safe. I have experimented, and the voltage at which people feel a shock has a wide variation, depending on where contact is made on the body, and on the skin resistance of the individual. If you use dry hands, most people don't feel anything below 20 volts. I knew an electrician who could not feel an ac voltage of over 100 volts. But if you lick your lips, then touch them to a small 9 volt battery, you will feel a tingle.
If you want to experiment with this, then don't ask people to hold live wires with both hands, as shocks which go across the body are the most dangerous. The contact points could be one hand and a point on the same arm. Dry skin has a VERY high resistance and even shocks from AC mains voltage at 240 volts (UK) are rarely fatal, because people will be wearing shoes and socks which are also made of insulating materials, so the current that flows is very low. The danger with low voltages is from powerful batteries which can deliver a huge current if there is a short-circuit, possibly causing burns or a fire. If you are using lead-acid or lithium batteries for amateur electrics, make sure each circuit is fused or has a breaker.

Dear. In English that I understand your question makes no sense. None. Using AC as a level sensor? Less sense.

at 12 vac i get shock,
obviously not fatal

What is a "child woods"?

15 volts AC is pass the limit. Even 9 volts will give a shock.

Besides the issue of a safe voltage, there are the questions of safe installation of the whole system. Where is the 15 volts coming from? What if there is a failure or two in the system?

There are mechanical water level sensors, much safer. Why not use one of that type? Professional system can use 24 VAC, but those have double or triple safety systems and cost a few hundred dollars.

UL says that any voltage over 42 volts is considered "high voltage".

TRY IT ON YOURSELF AND DO NOT POST SUCH SILLY QUESTION OVER HERE, YOU MAY FACE CHARGE !