What type of rockets are they? If water or air-powered you could probably go up to Riverview Playfield, which has nice open space. If they use the chemical engines, you’ll need to get out of the city to fire them off, away from the glide paths to Sea-Tac and Boeing Field. When my kids were into this, we went over to 60 Acres North Field near Woodinville. That is a haven for model rockets, R/C aircraft, drones, etc. Plenty of space for everyone.