According to the package leaflet for Solvetussan:

If you take more medicine than you should, talk to your doctor or pharmacist immediately. In the event of an overdose with clinical symptoms, the physician will immediately initiate symptomatic treatment and, if necessary, apply the usual emergency measures (gastric lavage, activated charcoal, parenteral fluid supply, etc.).

If the child is given too much, the child should be closely monitored. If you experience any worrying symptoms, you should urgently contact the nearest Night and Holiday Health Care. You can also take advantage of teleconsultation, e.g. via the Gdziepolek.pl portal:

https://www.gdziepolek.pl/telekonsultacja/z-pediatra

I'm attaching an additional opinion in a similar inquiry about syrup with the same composition:

https://www.gdziepolek.pl/opinie-i-pytania/39990/levopront-co-zrobic-jesli-dziecko-otrzymalo-na-raz-podwojna-dawke-syropu-levopront

I'm attaching an additional article about dry cough:

https://www.gdziepolek.pl/artykuly/kaszel-suchy-i-mokry-jak-go-leczyc