Can we freeze our baby's solids in advance? Are there certain foods that we should not freeze and must be kept & given fresh?

Yes you can prepare solids in advance and freeze them. As baby is not taking in a large amount of food yet, so freezing make it more economical and time savings. Most of the vegetables and fruits can be freezed. You can buy a tray/cubes to make the freezing and defrosting easier, e.g. Beaba Food Freezer Tray is recommended.