wow, you have a challenge, you can't use manure as fertilizer, well that's kind of like telling you that you can't have a garden. it is still possible, but more expensive and a bit harder. since what you have is more like a demonstration garden for kids to learn then maybe you can justify the expense of store baught fertilizer. my favorite is Dr Earth. I have taken several plastic watering cans and drilled out the holes large enough to let the grains of fertilizer go through with the water (about a 1/4 inch), the side benefit is that the watering goes much faster. I basically make tea from the fertilizer. first, when planting starts or preparing a bed, I add dry fertilizer with a small spoon, below where I plant the plants and then to the water before I water, about every 2 weeks. sandy soils tend to be acidic, so you want to check the PH of the soil and see if it is acidic or basic. sometimes yellowing comes from the nitrogen being locked up in the soil from too much raw plant matter, which acts as a sponge to the nitrogen/potassium. if you have been adding lots of mulch that has wood chips in it, it may need to break down more, or maybe this year its finally broken down enough and you'll have a banner year (fingers crossed). a lot of municipalities that accept yard waste get a little impatient and don't break down their yard waste enough to use in gardens, accept as a mulch. it's best to buy the mulch the year before you need it, if it's got a lot of wood chips in it, and let it compost some more in a pile. You can add unscented ammonia as a source of nitrogen to speed it up and turning it will help keep it aerobic, you'll have to read up on the dosing and experiment a bit to get the hang of it.
the best ready to use fertilizer I've found is Dr earth, I'd go with the 25 lb bag of Tomatoe, vegetable and herb, or maybe two bags of it. I use it as an all purpose veggie fertilizer. I then repack to ziplock bags since nitrogen is volatile and can evaporate:
Dr. Earth 733 Organic 5 Fertilizer, Tomato Vegetable Herb, 25-Pound . for the plants that are acid lovers, like blue berries and raspberries, you can use a rhododendron mix.
another more expensive but cool option is to get a jora tumbler composter, it's insulated and capable of getting the temps up to 130F or more, enough to zap just about any microbe that would be dangerous for kids from animal manure. I have this kind of composter and create about 1,000 lbs of compost a year, since it dramatically speeds up the process, making soil from chicken droppings (I raise about 70 meat birds a year) and kitchen waste in just 3 weeks. it's really an amazing system and I'd be swamped without it. there are days it's steaming so much you can see plumes of steam coming out the vents.
https://www.amazon.com/Jora-Compost...1517593126&sr=8-2&keywords=joraform+composter
I never tested the soil because I just assumed the miracle grow mix was what I needed. 
