I can get a high tension Lorry/truck tie-down for a fraction of the price of a Slackline kit, but would it be able to stand the tension required. Thinking about doing a bit of slacklining with the family, probably under 10 Meters, Dont want my daughters getting smacked in the shins by a ratchet!
Thing is broad webbing on its own, to make a primitive set up, is still actually pretty expensive, by the time you add some Karabiners (want to have a dedicated set separate from my rack) Its almost worth buying a £40 Slackline Kit.
I've got a mate who uses one (like you - family/basic use, no fancy tricks!), and haven't come across any problems.
I've also seen them set up for group use with the ratchet round the back of the tree, rather than in free space - assuming you can (1) protect the tree adequately and (2) tighten it OK that seems to take out a lot of the risk of kids being whacked by the ratchet.
I can't see there being any problem. I've seen the exact same bits of ratchet/strap used in a variety of creative ways before, including tensioning guy lines for a swinging trapeze in the circus, with no issues.
A branded slackline is probably made in the same factory as a lot of the ratchet straps, just with some logo printed on the webbing, aside from that they are identical. I got mine on ebay, they come advertised with various tension ratings, mine is something silly like 10 tonnes and is more than adequate. Cost about £20 delivered I think and that was with a 1m web on the ratchet with a loop at the end and a 15m web with a loop on one end so I didn't even need to cobble together any extra bits to mount it.
I've got the exact set-up and it is perfect! Perfectly strong, very strong in fact. The ratchet is rates to 5 tonnes! 20m line for £16 on ebay 2'' though. I use to 3m lengths of static rope to tie to whatever anchors you have with a double fig8. It never has or will unhook. The hook is bent through nearly 180-degrees so is well secured. Total set-up can be <£20