Hey there!
A child likely hasn't lived long enough to experience and take into account the limitations on life while adults have. This makes it to where adults allow these limitations to seep into their thoughts and ideas, giving them a limited scope of what they consider. For example, one of these limitations is self-control, which prevents crazy or impossible ideas from being thought up or expressed.
Without limitations, children can create some wild ideas and use their imagination to its fullest extent. Creative thinking requires one to assess all possible solutions and outcomes (no matter how outlandish they may be) for a situation instead of only expressing ones that are reasonable or viable. This can lead to someone else taking that idea and reworking it into a viable idea or solution.
TL;DR, children think creatively without limits. Creative thinking requires the same thing of thinkers, no matter how old someone is.
Hope this helped you out! :-)