This is one of the best Lake Michigan beaches for families. North and Oak Street are great for young adults and people without kids, but this beach is much more family friendly. If it is a nice weekend day, it will get crowded by late morning so plan accordingly.
I love this beach in the city. It's usually packed with families bbqing in the park area and covering the beach area. If you haven't lived in the city, you would usually go to one of the beaches farther south (like North Ave or Oak Street) but this one's where it's at. That being said, it still does get crowded as the day goes on, so if you're driving you want to pack a picnic lunch and make a day of it and head out in the morning.
We used to live up in this area of the city before moving to the burbs. Foster Ave. Beach was a great family beach. There is a concession stand and a large sandy beach. there are also lots of areas for grilling (bring your grill) and it's right along the bike path. There are great open areas for flying kites too. Seems to be more family oriented than some of the more popular beaches downtown.