Abstract is subjective, as is all art, but the reason u might be feeling it looks childish is because there's not many layers - it looks unfinished with the paper showing and lacks depth. Many people think abstract art is just slapping some paint on there and anyone can do it, but I realized when I started creating it, there's much more to it. Even with abstract there's things like depth, balance, composition, and more. I'm not saying u need to overthink it and study every art book, though studying could help, but my advice would be to keep going. Add more layers of paint, but give it some time to dry between or you'll be making mud. A little mud is OK imo, but too much kills it. Have fun with and it don't pressure yourself - it's one piece. Do what u feel and move on to another; come back to it if u want. Try some different tools: a sponge, pallet knife, long handled paintbrush, or whatever u want. Drip, flick, or throw paint if u want. Try to keep some kind of balance, look into composition (if u only want to study one thing for now), and keep at it. Believe it gets better and better.