My son takes Norwegian Gold First Omega (you swallow it), but I'm not sure if it helps or not. I wouldn't say we've seen any remarkable difference in attention or focus with using just fish oil, but it can't hurt.
We also use a supplement called Liquid Health-Attention and Natural Calm (which is Magnesium Citrate). It's made by a company called Natural Vitality. I'd check with your doctor before trying any of these, but we do see a difference when we are consistent with this regimen. I'm sure what's appropriate for my 9yo son wouldn't be the same for your 5 year old dd.
If constipation is an issue at all, the Natural Calm helps with that. I know there are folks who are big believers it the connection between ADHD and digestive/gut health. So, maybe there's something to it? Many of my friends who have children with attention issues also report that their children have digestive and/or constipation problems.
I also think exercise and any "heavy muscle work" like pushing, pulling, climbing are very calming for the nervous system. My ds9 is having some testing done soon, and we plan to hit the playground for a while before going to the testing. If school didn't start so early, we'd do that every day before school!
FYI, my ds9 took stimulant meds for years, but they no longer work in the low dosages he can tolerate. If we try to increase his dosage, his anxiety is significantly exacerbated. So, we're learning a lot about alternative methods these days.
So, probably more info than you were looking for! Sorry!
HTH a little!