My previous GSD, I could take his food or treats any time I wanted and he was cool with it. Never had any problems with him. He was very gentle/careful with his mouth. The current 8 months old...not at all. I've been bit twice now. He has drawn blood. He growls at me if I don't put his food down fast enough. If I make him wait for it, he goes nuts. If I get too close while he eats, he growls. If I reach for his bowl, forget it, he'll snap at me. I give him bones (or any high value treats) and when I go to take it away from him, he growls and shows his growly face at me.
I counter-condition foster dogs with this behavior to expect good things from me where food is concerned.
I may even feed them out of my (closed) hand, if it's safe to do so -- part of the meal, and we practice being gentle (I open up a flat hand when they're gentle and even teach them the word "good soft" or "good gentle" as a marker for what's right -- so it
is useful as it's a way to teach them to patiently wait for the hand to open and not chomp on it). I have sometimes had to wear garden gloves for this until they figure out the "gentle" part. Nobody's ever broken skin though....but I choose which dogs to do this with. Once they learn "gentle," you can use it with treats, toys, etc. and they should soften up and transfer the idea beyond the handfuls of kibble.
Otherwise, they eat in the crate, with the door closed -- safe and sound without any worries of pesky interference. They deserve a secure, calm place to eat. (That solves the kid problem too...as long as the kids aren't opening the crates and joining the dog in there...)
I also train all my dogs to sit-stay when I have their bowls at feeding time. They go to the bowl when they make eye contact and then get released. Growling would
not get rewarded with putting the food down -- none of mine have done that, as we practice NiLIF at meal time. One of mine dances and bounces on the way to her feeding spot but then she dutifully drops into her sit and waits patiently -- she knows that bouncing now is over, and she has to do the right thing to get to go to her bowl quickly...and she wants to go to the bowl quickly. She started out as a jumper who would crash into us when we were carrying the bowls, sending kibble everywhere....so we needed a solution. This is WAY easier than kibble flying everywhere.
When a food-guarding foster is eating in the crate, they should have some peace and security there. No other dogs or people harass them -- period. At most, I randomly walk by and drop in a handful of super-high value meaty treats in -- better than kibble, so me showing up means "JACKPOT" even if it's during a meal -- no talking or fuss about it, I just drop it in and keep walking. They quickly associate me showing up mid-meal to mean "more good stuff."
For toys, we work on leave it -- there's a dozen ways (at least) to teach that, widely available online. "Trade up" can also be useful if they really don't want to let go (so they end up with something better). Proofing the down stay is also a really good way to solve the toy chomping -- they can't get the toy until they're released and your hand is out of the way. During fetch, I trained my most excited ball dog (who was a chomper) to drop the ball in front of me -- if he tries to grab it again after he placed it in front of me, my foot goes on it (possessing it) and he goes into a down-stay. I can then pick up the ball, hold for a bit, release him and throw as his reward.
The issues you're describing are common -- but also pretty fixable. Don't give up! Please don't straddle him while he's eating though, as that very well could make the situation worse.