To make the tiara:
Start with a foundation chain. I chained 66 to fit the circumference of my daughter's head. You can chain in multiples of six until you get the right size (smaller kids will need fewer chains, for example).
Once you've got the length you want, join the chain with a slip stitch, be sure not to twist the chain.
Then hdc three rounds. I joined each round with a slip stitch, rather than crocheting in the round but you can do either.
For the crown part, chain two, skip two stitches and 3 dc into the next stitch, chain 2, then 3 dc into the same stitch. This will create the crown effect. Do this all the way around and join with a slip stitch.
There are several ways to do this: Here are a few other patterns/links. You can adjust any of them to get the look you want.
Royal crown patterns for kids