Yes, Santa Monica is a great place for families! It has a huge, fun beach, vibrant downtown areas, and plenty of kid-friendly activities. You'll be able to lounge on the beach, visit the famous Santa Monica Pier, bike along the beach, stroll 3rd Street Promenade, and play at many children's museums and parks.
Newport Beach has strong rip currents and sand bars, making swimming dangerous if you're not prepared. Ocean safety is extremely important. Key safety rules include knowing how to swim, never swimming alone, and avoiding the water if you're uncomfortable.
They're both safe. However, Santa Monica is its own city and has a constant police presence at the Promenade and The Pier. Venice is part of Los Angeles and so the budget is spread thin. Also, The Venice Boardwalk is empty and dark at night so doesn't feel safe like bustling and bright Santa Monica.