I've had a quick look on their site and can't find the hose, but both my MA stores, and the independent LFS have a selection of sizes on large rolls, so I know that they stock it.
As for kids in the store, I know that most parents try very hard to keep their children behaving impeccably at the shops, because they care about the welfare of the fish, etc. However, don't let it ruin a visit to the shop for you.
One of the funniest things I have ever seen was at one of the quieter branches of MA, where I've got to know the staff quite well. During the school summer holidays I was chatting to one member of staff at the till, while the area manager was working in the office, and another member of staff was just heading to the office for a break. A fairly regular customer came in with his 4 kids (3 girls and 1 boy, all under 10), and the chap I was talking to went to the fish tanks with him. Within a matter of seconds it was like someone had released the demons of chaos, who scattered to the 4 winds to destroy the universe.
The dad continued his discussion with said member of staff as if nothing was happening. The kids ran riot around the store, picking up items, depositing them elsewhere, running up to the guys and asking questions, then dragging the member of staff around the store by the hand asking "what's this for?". They also asked him his name, then chanted it at the tops of their voices whilst running around. I was leaning against the counter and even I didn't get away without being asked several questions about items that the kids brought to me.
At one point I turned around to see that the boy had got a large net on a pole off the display, and had climbed on top of the pond fish enclosures, and was trying to net fish. I will admit that my response was to get my phone out because if this kid fell in the pictures were going to be hilarious, at which point he climbed down, put the net back and ran off to do something else.
After the longest 10-15 minutes in the life of the poor member of staff, the family left the shop.
The manager and other guy emerged from the office, and were asked why they didn't come out to help. They said that they thought he was doing fine, and it turns out that they were watching the whole thing in the office on the security cameras, and having quite a chuckle about it.
The lesson here is that even if your children cause a bit of mess there is a strong possibility that the staff are either just taking it in their stride as part of working at an aquatics store, or they have all gone to the office to have a good laugh as one member of staff tries to deal with the entire situation on their own. Either way, try not to let it interfere with your shopping, the staff are used to it.