
Hi sillysue, here is the no-help answer...it depends! On the character/breed of the birds, for example. I have a mix of large breed and bantam birds (Welsummer, Light Sussex, silkie, Warren, Leghorn, Dutch bantam and black rosecomb) and for years I didn't separate the hens and chicks from the others--there were no casualties, but then my silkie cockerel is a very sweet, proud and helpful father. My Light Sussex cockerel is a huge bird, but he is a kindly bird and never minded chicks skipping about under him. My Warrens however are less pleasant birds with a strong cannibal instinct (not my choice to have them, they escaped from some other hobby keeper and spent days outside mine wanting to get in with my birds, I didn't have the heart to make them go home) and I wouldn't trust them with very young chicks.
I started separating hens and chicks last year simply because the pen got much larger, I had a lot more birds (including call ducks) and I had the space. The easiest thing to do is use strong cane or lightweight posts to support a temporary pen using a short roll of small gauge Netlon--I make mine out of leftovers, and they can easily be dismantled and re-erected as needed.
The chicks can easily be kept in for a couple of weeks in the horsebox and if your hen is an ex-batttery bird all that space will feel like luxury to her. You can pen them out after that but make sure they have a safe space to run back into--building a temporary enclosure at the back of the horsebox would be perfect.
Give them 2 weeks in the horsebox, then gradually introduce them with a temporary enclosure for 2 weeks and at a month old they should be fine for holding their own.