https://www.searchlock.com/search?sa...ercraft+models
Google image search [London Papercraft models] This throws up quite a few simple ones. Just click on a picture then select 'Visit site'. Mostly that will get you to where the download is offered.
Word of warning ... Using the word model in a Google image search will also bring up a different type of 'mode'l that you may prefer your youngsters not to see.