Thank you for reaching out to ask2.extension.

Personally I would not use roundup.  With young trees it may be possibly to damage them.

My suggestion would be to hoe them out or pull them by hand.   You can use a pre-emergent  or preventative weed product that will help keep weeds down. 

You may want to remove some of the mulch and check the status of your fabric.  Maybe it will have to be replaced.

I'm sorry, I thought the trees were new. You could try spraying the weeds around them. I would still be a little nervous because you just never know and even if your fabric is a year old dirt and stuff lands on top of it which allows the weeds to grow it doesn't take much for them, I just caution advice if you're going to spray.  Use the pre-emergent product before you put down new mulch if you want. Sometimes it works through the mulch, but not necessarily always. You will still always have some weeds come through, but it cuts it down.

As a volunteer I’m not allowed to suggest specific brands.  Just be sure to read up on the herbicides before using. 

There is a natural herbicide that is somewhat more expensive-corn gluten (powder or granules).  It might be worth checking out.

Be sure to follow all instructions of the product you choose.