3. The vaccines use completely new technologies
The Covid-19 mRNA vaccines are completely new, gene-based technologies (the body’s cells are given genomic instructions to reproduce part of the virus, rather than the standard process of presenting a weaker or inactive virus).  These have never before been approved for use in humans. 
They have not been licensed – they are only approved for temporary use.   Phase 3 trials have not been completed; hence the vaccines are still experimental. 
They are very different from the traditional childhood vaccines that have been around for a long time, and for which we have plenty of safety data.
We still have a lot to learn about these new types of vaccines.
It’s safer to wait