I would use the washable trunks. At my daughter's swim lessons, they aren't even allowed to use Little Swimmers because they've leaked and then they have to shut the pool down. I'd hate to make a mess in a Disney pool. If my daughter is going swimming and I think she might go number two, I'll put a little swimmer under her washable diaper. This helps with clean up because I know it can be a huge mess with a washable diaper. I'd rather use both and be safe.