Hello and welcome to the forum!!
I assume you are giving that amount three times a day? Are the lambs large/medium/small? Are they finishing off their bottles ...... Most people can only manage three feeds a day, or at the most 4, due to work, so don't worry. The amount you are giving sounds more than enough to me, althought as they grow they will want more of course!!
You are right to be cautious about putting them out in the paddock full time, they need to be quite big, as they have no mum for protection. It is best they are put in at night, but if its a nice day, no harm in them going in a little garden area, so long as they are safe there.
With regard to poisonous garden plants, most evergreens, in fact most garden plants can be poisonous to livestock. Laurel definitely is, and fir trees, yew, privet hedge etc. Daffodils too. I would keep them out of the garden if at all possible, if you think any of the above are growing there. We have honeysuckle in our hedges, so our animals have access to it. I would not be happy if they ate a lot though.