Tuna for your gal is fine-she would have to eat a truckload of nothing but Tuna,keep an eye on her poo's-with our gals they may have a loose poo with the first afterbirth poo they have (due to ant afterbirths they may have eaten) but after 5 days i'd be concerned.It is normal for her to take time out,feel her sacks and give a gentle squeeze,you should see milk come out,and as Wendy said feel babies tums they should feel full and have some weight to them
Stay away from the kitty milk as this will make her loose-give her water with a tiny amount of sugar and salt-help give her some energy as she may be feeling tired
Also to feed her try some raw beef(great for nursing queens) also an egg yolk with a little carnation milk to 2 prts water and a touch of honey,mix together and see if she'll have it-this is great for nursing mums-puts a bit of weight on them and encourages them to eat and also gives them a boost of energy
Sardines is fine too
Definitely keep her and babies seperated from other cats until babies are older too-and her blankets/food/water bowls and toilet trays