very slow growth

My very first successful spawn was last June, I was on holiday and my daughter did her best for me , and I had 10 survivors, so they're 8 months old or so, none of them are big (the second spawn are twice or more their size).

A couple of them are about the size of a 10p, they look healthy eat like gannets but are soooo small, am I doing something wrong or are they just very slow growers??? I realise as they are now in a community tank they have to fight for food, but they were on their own previously and got fed 2/3 times a day. The smallest ones are chocolates, not sure if that has any bearing on it?? second spawn zebras (a month later) are almost 3/4 adult size.

10p body size is certainly very small for an 8 month old fish. Although some varieties do grow faster than others, I'd expect even an albino to be larger than that at that age. In general I find that smokeys & chocolates grow faster than most!
Obviously, it could simply be that they were stunted by less than optimum feeding when your daughter was doing her best, and being in a tank with other, larger, fish will also have a negative impact.

Blue, pearlscale and perhaps to  a lesser extent blushing seem to slow growth, so if the first batch have any of these traits in addition to smokey/chocolate, that might be an explanation.
Another possible explanation is that the first pair may be closely related, in which case inbreeding could be a factor, also size of parents might be significant, if both parents were quite small, you might reasonably expect smaller, slower growing offspring.
 
It would be interesting to try and get a second batch from the original pair and see how quickly they develop assuming that you can time it for a time when your not going on holiday.

In general the key to maximizing growth rates is, regular feeds with good quality food, Frequent water changes every couple of days (but, in general, no more than about 20% at a time) and give the youngsters as much growing space as possible, with larger batches it really pays to separate the different sized fish, move the larger fish to a new tank and the next largest will really put on a growing spurt, such that you'll need to separate them out 3 or 4 weeks later!  

Very interesting Rob thank you, the parents had paired in the LFS tank, both smokies but one is a leopard, so they could be siblings I guess? I've had two more spawns from them since and some of those are near adult size, so it does look like its down to their first few weeks, I'll have to see if I can find another tank and put them on their own and see if they recover, otherwise I'll just have a couple of mini angels lol