Hey Neil!
I assume that /personalise-toys will be a more general category that will link to new more specific categories like /personalised-toys-for-boys right?
If that's the case, I would just change the content on the more general category to the new content/products that you have with the new site (rather than 301 redirect). The equity would be transferred to the more general category which lists products relevant to it. So if someone Googled it, they would still be able to find what they are looking for. The equity that you've built up with the more general URL will be passed to the new URLs that you create and link to (I assume) in the new iteration of the website.