We managed a weekend on a Parker 21 with four adults and two children many years ago. That took some good planning. We have a Parker 275 now but the answer will be similar to what you can expect on a Seal 26.
Sleeping: 2 in the bow, 2 in the main saloon and one aft makes 5. Already cosy but you might squeeze in another child in the aft berth.
Underway: 4 feels like the most you really want in the cockpit. You could sit one or two on the roof in fine settled weather.
In summary, with four on board you can set sail and enjoy weeks on end. Five would be ok to accommodate friends visiting you in port.
Ken Surplice