As someone who works in a tangential field, the following advice is for England, but I can't imagine it'll be too far from the situation over there. We have recently been doing something similar with one of our own parks trying to reclaim it and revitalise it.
Here's what we'd advise:
1: Drugs are a police matter. Every time you see a drug deal, submit a report. Usually in English forces there's a form to fill out on line to do this. This means that suddenly there's a cluster of reports. It's the old thing of one report saying '... and this is always going on!' isn't going to get noticed. 20 of them are, as it simply makes the stats look bad.
I say do it online as it means you'll not tie up call handlers.
If things get ignored then there should be a complaints procedure, and if that fails do you have a Police Commissioner? This is what they're for. But don't do the complaint stuff too soon, give it a lot of time.
2: Contact the council Anti-Social Behaviour team. They will likely be in contact with the police, and may have soft measures they can put in place. Something as simple as Enforcement officers doing periodical patrols in the area can make it seem less inviting to the miscreants, even if nothing comes of the patrols, a uniformed individual bimbling along does make the drug dealers/users nervous.
To help with your case before you contact the ASB team do a log. Take obs over two weeks and note times of gathering and drug dealing. No need to include photo's though.
The council can spend effort on making the place more inviting to people. Lighting, CCTV new/cleaned up play equipment, removal of bushes that provide cover etc. All of which needs money, and to get a council to spend money you need your councillors on board, so start moaning at them. Try all the councillors whose wards abut the park, not just your own. Or failing that councillors in wards which are within walking distance.
Don't expect instant results either.
