It's really going to depend on the personalities of the children whether they get scared or not. I think the arch will be fine because it's not very threatening; sure there's a grim reaper but the arch doesn't move or make any scary noises and kids will be so bewildered at the size, color, lights, and noise of the air pump that they should be fine. The mummy may be a little scarier because of the eyes and the movement. Kids that young usually don't understand it's a prop and may be fearful. Your best bet in doing a haunted house for kids that young is choosing props that are interesting but not threatening. Don't pick anything that makes really scary noises or moves in a threatening or distressing manner because the kids may think it's real. Make sure your "haunted house" is well-lit with no dark scary corners. Use lots of colorful, well-lit props, like funny-faced jack-o-lanterns with multi-colored fake "candles" burning inside (much safer than a real flame), or depictions of classic "monsters" illustrated in more child-friendly styles. Jumping out and startling kids will only make it scarier; it's better to set it up more as a "tour" where they can walk through and take in the atmosphere. They'll be so entranced by everything going on that you really don't have to sweat the spooky stuff.

Here are a few examples of decorating items that would go well:
http://www.walmart.com/catalog/product.do?product_id=10543317
http://www.walmart.com/catalog/product.do?product_id=12335888
http://www.walmart.com/catalog/product.do?product_id=11974441
http://www.walmart.com/catalog/product.do?product_id=11026514
http://www.walmart.com/catalog/product.do?product_id=11974418

Give each child a reward for being brave and going through your "haunted house" to help them associate the activity with good feelings, and above all else, make it easily escapable should a guest get too scared and want out immediately. Have a happy halloween!