I used to struggle with these kinds of feelings, too. They're totally normal, and working through them is a common part of the deschooling process. One of the side effects of schooling and the efforts of parents to teach kids "right" ways to be, is exactly this kind of cynicism or ennui toward things that light up children. We use the word "childish" as a put down, frown when they delight in the "wrong things" and are instantly suspicious of their interests. One of the truly wonderful things about unschooling can be learning to let go of a few of those layers of hardness and relearn the art of delight from our kids.
It absolutely does take an effort to overcome those ingrained messages that toll "this is stupid, stupid kid stuff" and see that joyfulness and wonder can be about the simplest, sweetest things - a pink plastic pony in a McDs kids' meal can be as precious and wonderful to a child as a bright new beetle or a river smoothed stone. And it's Okay to let our kids appreciate they world as they find it - to love color and sound and motion because that's who they are. To say yes to their delights as we strive to rekindle our own. It feels stilted and fake at first because we've forgotten how to play, told ourselves that playfulness is fake and cheer an advertising ploy to armor ourselves against the years of nos and limits and have tos of our own childhoods. We don't have to teach our kids that armor - they'll grow their own as they need it. We can give them gentleness and appreciation of their enthusiasm, support their delights, learn from them how to look at the world again with wonder and awe, and it doesn't take anything away from them. It lifts them up and lets them hang onto a little more joy for a little bit longer.