My Veggie Seedlings Are Not Happy Chappies

Hello and blessings to all.

This year I decided it was time to take back independance and grow a few vegetables for myself and my son to feast on. So far we have planted out lettuce, rocket and carrots (we have tomatoes in the house, and they seem to be doing well) but all the rocket and lettuce have grown well up to a four leaf point and now the have wilted, gone brown of leaf and died, we only have one tiny plant left!! I am a total novice when it comes to gardening and I'm sad because I don't know what happened or why my little guys perished. When I went out this morning to check the carrots, I was saddened again to find that half of them have disapeared since two days ago!!! The ones that are there still look healthy enough but I'm worried hat this won't be the case for long, they too are just at the stage where thir leaves are just developing beyond the two leaf phase. I'm desperately in need of some advice on how to rescue these guys, they don't deserve to die so young!! All the veg are planted in grow bags (when putting out i put extra drainage holes in the bottom, followed by a thick layer of gravel) and they are filled with peat free compost from b&q.

Please help, any and all advice will be taken with the most gracious thanks, we are also starting off strawberries, aubergenes and peppers and I would like to ensure I can look after all these plants in the way they need me too. Help help, and thankyou!