Oh dear, you saying you are going to bank transfer their deposit makes me think you haven't put their deposit in a protection scheme which puts you in a very precarious position. Here's some information from the government website:
Getting your deposit back
You can apply to your local county court if you think your landlord hasn’t used a TDP scheme when they should have.
If the court finds your landlord hasn’t protected your deposit, it can order the person holding the deposit to either:
repay it to you
pay it into a custodial TDP scheme’s bank account within 14 days
The court may also order the landlord to pay you up to 3 times the deposit within 14 days of making the order.
At the end of the tenancy
The court may decide that you won’t have to leave the property when the tenancy ends if your landlord hasn’t used a TDP scheme when they should have.
Also it is sad you didn't put a no dogs clause or a pets requite written permission line in the tenancy as it means you cant really legally complain over them having a dog. Saying that as a long term tenant with two of my own dogs I have always looked for homes without a no pets clause and then conformed with the landlord that it is ok for me to have my dogs before considering handing over a deposit or signing anything so I can see why you would be upset.
I do think as PP have said you may find selling the home difficult with tenants sat in it as it can genuinely be a very long term process evicting a tenant and if they are bankrupt they may be unable to move out quickly. I'm not saying they will deliberately cause you trouble but with children a dog and no money the council are really their only option and unless things have changed since I lived in brum (presuming that's where your house is) the waiting list is over a year long. Plus without issuing a section 21 they wont be required to move out if the council cant house them. Even after a section 21 is issued they don't have to move till you have gone through courts and sent bailiffs to remove them which as others say takes months. Again I don't think they will do this to deliberately cause you issue but I do think that it might inadvertently happen in their desire to not have their children on the streets.
Maybe have a polite, not threatening or aggressive word with them about the dog poo, explain they absolutely must clean it up, if no change tell them you will be forced to issue a section 21 to speed up the process of removing them if they continue leaving it.