What should I do with biting dog?

First of all, this is not our dog. We are looking after a colleague's 3 small dogs whilst they go on holiday for a month!

Today is day 2.

Two of the dogs we have had before for the same time period (and the third is a sweet youngster) One of the two from before was always a bit spoilt and grumpy. But this time it's a different dog almost, it's growling and aggressive just to me but to my wife too.

Not only that but it hasn't stopped barking either(but I can forgive that and I know it will improve)

This morning it wouldn't let me in the kitchen door (stairgate) , it wanted out and I wanted in.

Naturally I tell him, no. He growls. (Not good!) I'm not sure what to do now. I decide I've got to just open the gate and go for it, stopping him from getting out in the process by using my lower leg as a barrier.

As soon as I blocked his path he nipped at my leg. I push him backwards with the same leg and he grabs my slipper , taking it off my foot almost. I've managed to shut the gate by now I'm in and I thought that was the last of it, nope! He came straight back to me for more, more biting and holding on to clothes etc

We have 3 kids in our house, the youngest is 6. I now fear the worst may happen with the kids, I can't just do nothing and then act surprised when my children have been bitten.

So what do we do?

We can't afford to kennel it (I'm happy to keep the other two) for a month! The colleague has been useless about it, I've asked if he's got an alternative accomodation, nothing.

I'm livid , this is not the same dog as before , it's clearly got much worse.

My only thought was muzzling it, so that's what I'm doing after the school-run, going to the pet shop for one. Yeah? I hate muzzles too, trust me

I'm sure someone will ask but I'm not sure it's relevant, they're shi-tzu's.

I'm going to accept the colleague's claim of " he's never done that before, he's probably just really upset" but obviously , I won't ruin his holiday but we he picks them up, I'll be telling him never again.

As a parent you just ignore a dog that bites in your house, regardless of it's physical size.

What would you do in our situation?

First Edit: for what it's worth- this is a paid venture, it's not a small amount either but not enough to cover an official boarding kennel and we don't have the paperwork for that either.