Welcome to Townhouse, Stephen. Thanks for sharing your story, it was so much fun to read. I think you've got just the right tone. Humorous and not too scary for the little ones. I loved the idea of the wallpaper animals coming to life, and I think kids will too.
I think this poem could appeal to a wide range of ages, especially because of its length. I'd recommend upping the girl's age to 6 or even 7. I assume the brother is younger, this way you have all ages covered.
Although, if you look hard, I think, From time to time, you’ll spot them wink).
I thought this was a great way to end the story. Upbeat and fun. Leave them wanting more and wondering what the next adventure will be! (don't like the parenthesis, however. You don't need them.) The next lines come across as preachy and kids will turn up their noses right away. Stop while you're ahead! I'd eliminate the last two paragraphs.
These are just my opinions so take what makes sense to you and leave the rest. I'm looking forward to reading more of your wonderful stories!