Guess it is easy for her to think that you have given stuff to her whereas you think you have lent stuff to her... tricky because you both feel like you are the one in the right and the other one is being unreasonable.
For what it's worth, I don't think YABU; just that maybe next time things need to be clarified in advance.
Does she use the equipment you lent her as well as the clothes? Could you steel yourself and say 'I know it is inconvenient for you but it is inconvenient for me too - you know I had promised my friend she could borrow my stuff when you had finished with it and then you know how things are, money bit tight etc, I need to sell it ready for buying more stuff now that the boys are getting older.'
See how she reacts - do you think there is any chance that she has sold the stuff herself?
If she says it is difficult to get out of the loft, then volunteer to be there to look after her ds while she/her husband get the stuff out of the loft. A pain for you I know, but then you would at least get it back.
How much longer until your friend is due - what is the urgency? Can you remind her every time you see her and ring her up every friday night /day before you & she have a free day to be able to sort the stuff out.
Tell her directly that, if she has another one and you haven't sold it, then she is welcome to borrow it again. BUT you have promised it to your friend and she is putting you in a very difficult position that you need to sort out. If she dodges the question more than a couple of times, ask her directly if she is refusing to give you back all the baby stuff that you lent to her and why this is as you don't think it is fair, and please could you have some money for it as she is not keeping to your agreement (as you will otherwise have to go out and buy some more stuff having promised it to other friend).
good luck, be brave and let us know how you get on. (remember to practice this discussion in your head several times before you have it so it is not too bad, making sure you have answers to all her possible different replies!)