Here's one for the ages: how on earth can you keep a curious new toddler out of an older brother or sister's stuff? Toy cars, too-big stuffed animals and even glitter are potential hazards in little hands.
With three kids under five, I know a little bit about the issues that arise from keeping curious preschoolers in close proximity to an infant. The other day, my oldest tried to feed my youngest kid a phone charger! Anything and everything in their reach is a potential battle sword, noose or cattle prod. Fortunately there are several tactics you can employ to keep baby safe.
Corral the toys. Basket, shelves, bags, boxes. Anywhere you can uncover the smallest bit of storage up off the ground is golden. I only bring down one play set at a time to ensure that I can keep track of all the odds and ends. Better yet, teach the kids a lesson in responsibility by making them pick up their last toy mess before setting up a new one.
Lock up the kids. If you can't stash the toys in a closet, why not the kids? Obviously I don't really suggest that you put your kids in captivity, but assigning them specific "zones" can be helpful. My older children understand that building bricks and other choking hazards are never supposed to leave their playroom. For younger kids, baby gates can help keep them and their toys within a safely confined area.
When older kids can't be contained, providing the youngest with a designated "safe space" is the best way to go.
If older children are really being dangerous with toys and seem to be intentionally endangering the baby, it could be that they are bored and need a new outlet. When my oldest son kept bringing small items around his little brother, I eventually realized that he was advanced enough to need the challenge of a preschool. At a certain point, hanging with a younger playmate at home may not cut it anymore—especially if Mom and Dad are too busy to consistently come up with fun projects to keep everyone entertained.
Banish the baby. Stick him in a bouncer or walker, or let her roll around in a play yard. When older kids can't be contained, providing the youngest with a designated "safe space" is the best way to go. Especially when the littlest one starts curiously crawling, offer up a corner with things that are selected specifically for them.
The challenge with kids at different ages is that their developmental stages will always be staggered. Once you finally get the hang of one phase, they'll quickly be on to the next. One of the best ways I've found of dealing with this constantly transitioning life is to befriend parents with kids of similar ages. There are always new tips and tricks you can learn by observing others. If nothing else, they can commiserate with you!