There is a difference between not trusting your kids and protecting them from inadvertently coming across unsuitable material.
I see things like content filtering as part of a range of measures that parents should be using including education and monitoring.
Its not really a matter of trust - Kids are simply curious about anything and everything. Once their curiosity is aroused about something a large number (and increasing) of six or seven year olds are savvy enough to be able to use search engines to satisy that curiosity. Its just one of the consequences of having so much information available at the touch of a button.
Content filtering etc, I feel is basically a waste of time. It makes money for the company selling it and may lull a parent into thinking they have done something. In reality kids can usually find ways around it in minutes if they are so inclined. I used to occasionally teach maths & computing to kids of various ages ranging from 7 to 15 and its suprising what they can do. It wasn't uncommon for even the younger ones to be trading methods to circumvent any form of filtering they came up against. Eight year olds could eaily configure virtual private networks and proxies in order to get what they wanted whilst ten year olds were more than capable of making their own bootable linux disks so they could boot the pc's into a system that had no controls whatsoever.
So, dont be fooled into thinking kids are protected just because you've put a filter in place. Chances are that the kids will get around that filter in less time than it took you to configure it. If you are worried then there is no substitution for allowing internet access only when supervised.
As a general principle, I would NEVER allow any child under the age of ten to have an internet connected PC in any private space (bedroom etc). I would always make sure they use a PC in a family room and usually only if an adult is also in the room.
btw, Kids under 10 are actually rarely interested in porn. If they do come across it then it rarely holds their interest for more than a few minutes before they move on to something more interesting like trying to download the latest music, video or game. Most of them consider porn to be quite "yukky" !