Tweak search function on osCommerce shop

We've put a version of the advanced search form near the main nav of the header of our site so that this functionality is accessible on every page. Users can select amongst various options including size, colour, price, brand, etc. to quickly find the products they want. Another part of the site's capabilities is having parent and child products. This is used primarily to list different colour variations of products - e.g. the parent product is black, but it also comes in white, blue and red. This functionality is required so that stock levels can be traced in the back end and so that different product pictures can be used on the front end. It's also important to note that each product (parent and child) comes in many different sizes. These search and colour variation functions need better integration. At the moment, child products aren't showing up in the search results if the parent product doesn't match in all other respects. For example, if a user searches for a small white shirt, even if that child product is in stock, if the (parent) black shirt doesn't have any small sized stock then it somehow cancels out the child product from the search results. This work needs to be done on two identical oscommerce stores that are hosted on the same domain. Note: There's likely to be some follow-on work on this as we're going to start categorising a large subset of our products by impact level (i.e. suitable for high, medium or low level sports activities) and so will need to add this as an option to the Quick Find search. I'd also be interested to hear any ideas about making the form (see attached screen shot) look more modern - perhaps some nice CSS and AJAX/jQuery effects?