I think the best thing to do is stick with Volvo and get yourself an XC90. Tonnes of space, mechanically long lived and they've been in production since 2003 so you'll be well able to find one at your price point (which should be around €14,000 or so). Three seats across the back will be no trouble and there's a bonus two extra seats in the boot.

 

There is another option though. You could upgrade to a V70, which might give you the cabin width you need and which would be cheaper to tax and run than the XC90. Take along your kids and their seats on a test drive or two and see if they fit. If not, there is a solution in a seat insert that will convert a normal two-plus-narrow-middle-seat-bench into three abreast anchor points for kid seats. Mooney's on the Naas road sell them and they fit most brands.