How safe are small backpacks for the day?

I like a daypack for most of the stuff you describe. It works great for small things that aren't valuable plastic fork, wipes, sanitizer, chapstick, led light, etc. I also keep some snacks or a picnic and a packable jacket, sweater or both.

I don't put a wallet or cash in it. If pickpockets target you, they will watch to see where your cash, camera or other valuables are stored. My daily cash is in a front pocket, large value is in my money belt. If I need to dip into my money belt for any reason, it's in conjunction with a trip to the toilet rather than in public.

My camera is a pocket size digital with a neckstrap. If I'm not taking pictures, I keep it inside my shirt, sweater or jacket.

Ultimately, I only worry about my front pocket.

Brad comments are similar to what we do. Keep your valuables on your body and have a lock on every zipper. The other issue is how you handle the bag especially in crowdrd conditions. For example- people will wear a day bag/back pack on their back in a crowded situation making it an easy target for someone behind you. Some wear the bag on the front but that is only marginally better. The bag is still at a level where someone can reach into it especially if you are distracted. When we are in a crowded bus or subway car the bag goes on the floor, if standing it is between us on the floor. When entering/exiting buses, or escalators, etc., I carry the bag around my knees, almost dragging on the ground. The idea is to get the bag out of the range of sneaky fingers. For the same reasons don't wear camera cases, etc., around your neck. You should be fine.

I always carrying a daypack with me. I use those little locks (combo)on the zippers-not that strong at all but a bother. Also in situations (crowds-subway-bus-commotion)I take it off my back and into my arms or at my feet.- I also carry it with my wrist through the strap at top and through the shoulder straps too. Keep aware and you won't be lucky ...you will be SMART Just to be clear---Everything of importance in a money belt that isn't dipped into in public!!!

If you have a lock on the backpack zippers then maybe. Otherwise I wouldn't do it. I know someone that left her backpack unlocked one time and guess what happened. Lost her wallet with all her money, credit/debit cards, and passport. This was in Italy too.

I use a small backapck all the time when I travel, but when I am in crowded palces I don't wear it on my back, but I hold it tight with my hand. I also purchased a day backpack made of a thick material and it has zippers that I can lock.
I usually carry my credit card and passport inside my jacket. I have a "special" jean jacket with big inside pockets I can fit many things in. I wear my digital camera many times hooked on belt, but when I don't need it for a while I put it back in my backpack.

I put my day pack on one shoulder with the wrong strap or both straps so it hangs kind of infront on me and then I wrap my hand around the bottom of it isn't so easy to get in to.

I like my messenger bag. I feel way more secure than I would if I were wearing something on my back. It sits toward the front of me so I always know what's going on and has many secure places that I'm pretty sure the only way you get into without me noticing is by clubbing me over the head.

A day pack (small backpack) can be stolen. That happened to a person I know. In a city in Europe, he was sitting on a bench outside, looking for an item in his big packpack which was beside him on the bench. His day pack was on his legs. The day pack disapeared, quickly. He did not feel the day pack being lifted off of his legs. He did not see the person who took it. That event was not at night. There were other people around him. His journal book, which he wrote in every day for nine months in Europe and Britain, was in the day pack. He felt sad because of losing his journal book. He could not remember most of what he wrote in it. ... If a small amount of cash money and one credit card will be stolen from you in ITALY, I suppose that would not be a disaster for you. But I do not ever put cash or a credit card in a day pack, or in a shoulder bag.
And yes, Rome and Florence are pickpocketers' paradise in crowds of people in June. And thieves are attracted to a day pack.