Assuming the child doesn't really have an opinion on the matter. We've asked her and she has no fixed ideas, says something completely different each time.
3 year old girl, I want to get her hair cut shorter because I find it quite messy and harder to deal with and I think short hair is cute. Plus it's always slipping out of clips and getting in her eyes and I think it looks annoying. It's currently down to around shoulder blades but is a bit curly so looks shorter when dry. She likes wearing hairbands and clips so I'd leave it long enough for that, but just more manageable.
DP is opposed as he thinks she looks cuter with longer hair. AIBU to ignore his opinion since he's hardly ever the one to wash, brush her hair or put it up? I always said that no child of mine would be allowed long hair until old enough to deal with it themselves. However I changed my mind and would do it for DD if she expressed a clear desire for long hair but she hasn't.