Plenty of people get immediate symptoms from contacts that are not concussions. Startle response can cause the symptoms.

What you likely read was 'A concussion cannot occur unless there are some immediate symptoms.' But, immediate symptoms to not mean a concussion. They can also mean a subconcussive impact or just a startling bump.

Another way to put it is, All concussions have immediate symptoms but not all immediate symptoms are concussions.

A 2 1/2 year old's head is too soft to impart any serious force.