Need help "porking" up babies!

As some of you know I recently adopted three kittens from my moms barn litters. They had goopy eyes but that is gradually getting better with regular cleanings. However Fluffernutter and Trotter and quite skinny. Fluff is the worst and is just all skin an bones. He eats very well with regular cat food but I was wondering if there was something I could do to help him add some weight just a bit faster. He is a very sedate kitty and snuggles but it breaks my heart to feel how skinny he is. Trotter isnt as bad but he could use some fat on his bone too. Does anyone have safe tips to "pork" up my babies..I heard about the baby food, pedialyte and wet food mixture but I can only do the baby food and wet food. Pedialyte is just too expensive at 9 bucks a bottle....Even the generic walmart brand is pushing the budget. But anyone with advice please pass it on..I dont want to make them into fat cats...as cute as it would be..but just something temporary to help them add some meat...

I used Nutri-cal from the pet store. It's a gel that cat's like. I guess you could mix it into their food as well. My kittens like to lick it straight from the tube. It's a high calorie gel and a little goes a long way. I think I paid 6 bucks for the tube and I have at least a half a tube left 3 weeks later. I don't give it to them every day, but it's more of a treat. I imagine you could use it every day.

Also take the little one by himself and give an extra meal or two during the day.

i would be wondering about worms and such, have they seen a vet yet?

I guess now Ill never know what happened. I had to post a thread in Crossing the Bridge because Fluff passed away last night. He was eating good and I thought he would have a chance to get better but he just deteriorated yesterday with no warning. We buried him today in the back yard with a nice wooden plaque. Its just heartbreaking when a little baby gets taken away so soon.