If some members want more frequent activities for their child, let someone else start a different club on another day. Figure the costs out ahead of time. Keep your theme or subject based learning club family-sized . Six to eight students is optimal. Twelve might work for very compatible groups.
Larger clubs can easily become classes and prone to developing behavior management problems.
Expect to spend about half a day or more each time you meet. The time you spend together needn't be all planned activities. Include some social time. Bring lunches to share, or potluck, or take turns providing snacks, or meet afterwards at a nearby park. Younger children need time to run off steam, older students need to huddle, and parents need to yak.