Toddler bed mattress?

[Stressedafff]

DD 3 can’t sleep a full night in her bed. She’s up and in mine after a few hours. I’ve felt her mattress and it is quite thin

It’s a toddler bed. I’ve not got the money currently for a new mattress so I’m just wondering if a topper would be apt?
Her bed is 140 x 70 and silent night do one for a standard single

Are they safe to use with toppers? She can’t stay in my bed as it’s a single and there’s no room

At that age I'd consider a bigger bed. Think our toddler bed recommended up to 4 years.
Our son had a thick toddler mattress but it was awful and had a lot of slack to it after 3 years of use. We got him a new single bed when he was 3.5 and his sleep really improved. He still woke up in the night but was comfortable enough to go back to sleep by himself.