Trouble w/ Zycom Bike Clamp
Assuming this is the bike you are talking about

Zycom My 1st ZBike
According to the assembly video on their website (I couldn't find a manual)
The fork and handlebars are held together with a clamp

And the seat post is held into the frame with a clamp

The two clamps look similar but might be slightly different sizes. Swap them back and forth to make sure you have the right one in the right place. If one clamp is a little larger than the other the larger one could be for the fork/handlebars leaving the smaller one for the seat post clamp.