This research project will investigate if a supplement containing a unique combination of prebiotics and probiotics can influence behaviour in children diagnosed with autism spectrum disorder. The study will use a combination of human milk oligosaccharides (HMOs, prebiotics) and probiotics as an oral powder.This clinical trial will have two consecutive phases. Phase 1A is an 8-week randomised, double-blinded, placebo-controlled trial. Participants will be recruited and randomised (1:1) to receive either the investigational product (treatment group, n=30) or the placebo (control group, n=30). Phase 1B is an 8-week open-label study. All participants that complete Phase 1A will move into Phase 1B (n=60). This allows all participants to receive the investigational product and will provide additional information on increased duration of treatment.