hello sir.i am from singapore,and i came across ur problem.don't panic,it's common for kids to do such things.all you have to do next time is lock it,remember the code,and give it to ur child..i also had this problem 3 months back.i went to a phone dealer near my residence,and they master reset the phone,causing the phone's unlock code to be disabled.It may delete ur phone data,but it is the best alternative.there are other methods,but they are costly.i fixed this prob for free.hope u will benefit from my suggestion.

By accident after play game in my phone,my son ,8 years old,was set security code and forgot what number he stored.Now,everytime I turn on my phone it say PHONE LOCK and PASSWORD.Can someone help me?My phone is Samsung SGH-A737 .IMEI 355992015041691