Younger children can trace over the lines of the Union Flag (better known as the Union Jack) before colouring it in.
Union Flag Tracing
Here's a high quality colouring page of the UK's national flag, the Union Flag (more commonly known as the Union Jack). Colour it in traditional (and correct) red white and blue, or go mad with other colours!
The national flower for the United Kingdom is, of course, the rose. Roses come in lots of different colours, so you can choose whichever you like for the rose on this colouring page.