PSB
You don't make it clear whether you're thinking about putting two children in one seat or just making a general enquiry, but the consequences of something as inocuous as a rejected take off at speed or, god forbid, a forced landing or an incident incurring similar G forces, don't bear thinking about.
At the sort of age you are discussing, children's bones and skulls are incredibly vunerable, which is probably why the practice is banned is some countries and severly restricted in others.