I agree with the others that criticising what they did for your child after it's too late for them to do it is not likely to be well received. Are you planning to/in a position to give them a gift in thank you for looking after your DD? If so, perhaps you might consider giving them a couple of puzzles that would have been appropriate for your DD and are more advanced than what they have now, maybe with a note saying "I realise these say they are aimed at older children but my DD was enjoying puzzles like this at that age and we noticed you didn't have any like this, so perhaps they will be a useful addition that will be enjoyed by other children" or something like that.
TBH I'm not really with you on what they did wrong, though; puzzles at the wrong level is one thing I can understand, but I have a hard time in thinking of what another three examples could be. I don't see it as the role of a play-based centre to "challenge" children at all - IME they challenge themselves and don't need special equipment to do so; the beauty of most equipment used with this age group is that it can be used in lots of ways. When I think about the things my DS most played with at the equivalent stage, I think of books - which were there for adults to read to the children, but which he read to himself - sand and water etc., outdoor play equipment, construction toys, art materials sometimes - all these things were used by all the children and I don't know what else I would have wanted him to have available; he certainly enjoyed it and learned from it. I wouldn't have wanted them to "identify" my DS and do special things with him. I'd have been upset if they'd been trying to force him to do things that weren't interesting to him - is that what happened with your DD? If so I'd think twice about sending another child there. If it's just about what toys etc. they have, then I'd think that budgeting to give them things occasionally (say, instead of buying them just for your own child at home) would be likely to be more effective than trying to get them to buy them.
