What can you give me? I’m interested in anything! 
A lot of people seem to love [name]Seraphina[/name] nn [name]Sera[/name]. I love [name]Seraphina[/name], but I’m not sure about [name]Sera[/name] as a nn for it. I personally love just [name]Sarah[/name], but if not…
My favorite option for [name]Sara[/name] as a nn is [name]Sapphira[/name]. The first two and last two letters together, voila! I think [name]Sapphira[/name] nn [name]Sara[/name] is adorable. 
Now that I think of it, [name]Serena[/name]/[name]Seren[/name]/[name]Sarina[/name]/[name]Sirena[/name] nn [name]Sara[/name] could work as well. I really love [name]Serena[/name], as well!
Well a gp of mine is [name]Serenade[/name]. Not sure if it’ll ever work well on a real child though.
Some other options:
[name]Sariah[/name]
[name]Serafina[/name]/[name]Seraphina[/name]/[name]Sarafina[/name]
[name]Serafine[/name]/[name]Seraphine[/name]
[name]Sarina[/name]
[name]Serena[/name]
[name]Serenity[/name]
[name]Seren[/name]
I wish I was more helpful but all I can think of is Sarabeth. I like @ashthedreamers suggestions though.
I looked up [name]Sera[/name]/[name]Sara[/name] names on NB (I’m sure you have too) this is what I came up with:
[name]Serafima[/name]
[name]Serafina[/name]
[name]Seraphine[/name]
[name]Sarai[/name]
[name]Saralee[/name]
[name]Sardinia[/name]
[name]Sariah[/name]
[name]Sarita[/name]
Are you pronouncing it ‘sah-ra’ or ‘sair-ah’?
I can only think of [name]Seraphine[/name]/ [name]Seraphina[/name] and [name]Seraphim[/name].
I also like [name]Sarah[/name]/[name]Sara[/name] on it’s own but others you could use are:
Masara – a magical woman
[name]Sarahi[/name] – princess
Sarala – straight
Sarama – kind woman
Sarani – path
Saranya – surrendered
[name]Sardinia[/name] – woman from a mountainous region
Sariel – beloved of god
Sarila – waterfall
Sarisha – one who is charming
[name]Sarita[/name] – flowing, princess
Sarvani – the all
Tasarla – morning, evening
Moserah – disciplined in learning
[name]Seraphina[/name] – burning one, fiery
Proserpina – to emerge – this ones a bit of a stretch but it has the SER and an A in there
[name]Serena[/name] – serene
[name]Serenity[/name] – serenity
Serilda – armed warrior women
Serotina – dusk
Servia – daughter of those who serve the lord
[name]Sarina[/name] and [name]Sarita[/name] are both elaborate forms of [name]Sara/name. [name]Sarai[/name] for [name]Sara[/name] doesn’t make any sense to me as the sounds change too much.