Age range is often a problem, many people find it hard to keep it "young" enough, but there's a neat trick to hit it (I think).
Don't think of it as something for tiny children to enjoy by themselves.
Remember the parent, too.
The age range you're aiming at with picture books is four-year-olds, and the thing they will love most is grown-ups having fun while reading out the book, because then they'll read really well.
Writing picture books is really like writing a script for an amateur performer (the parent) with helpful illustrations for the audience (the child). Write the book so you enjoy reading it out, and if you find yourself quoting it randomly in the day you are doing well, and if your housemates start quoting it back to you you're winning.
That's actually one of my favourite things to have worked out, and the main reason I enjoy making picture books - they aren't just to be looked at, they are meant to be performed, and how much fun is that!
But even though you might be writing to include a parent, the book itself remains the property of the child. Parents are invited, yet the actual core of it must address emotions important to the child - big things like loss and friendship and adventure. And with the pictures, you are talking to them directly - that's where you can bring in all the stuff that you know you would have got excited about as a child, whether your parents understood or not.