A relative clause in English often well translated with a participle in Latin.
Depending on which way you want to phrase it, you can use absentes (those who are not present) or praesentes (those who are present).
The simplest phrasing would be, I think, filios meos absentes amo, "I love my children who are away".
But the nuance needs to be changed a little, and we should also avoid sending the message "I love the absence of my children", which is a valid reading of filios meos absentes amo.
I would prefer to keep the phrasing concise, but of course not at the cost unintended meaning.
The simple idiomatic choice seems to be to say add et and say filios meos amo et absentes, "I love my children also when they are away".
I find this much more powerful than an unwieldy explanation with relative clauses.
There are ways to say explicitly "no longer" (such as non iam), but with children there is context: they are with you when young and move out when they grow up.
When a child is away, I find it implicitly clear enough that they were with you at an earlier time.
What remains is to pick the most suitable words.
I find filii to be the most suitable choice for your own children.
They are not just boy and girl but son and daughter — liberi sounds too general to me.
There are options for "love", for example amare and diligere.
I think these are the main two options and both are suitable.
You can also use both if you want to emphasize that you feel at your best in their presence and you esteem them highly.
I find amare alone to be a better fit.
You could also structure the whole thing differently and say something like fili mihi carissimi sunt ("my children are very dear to me"), but I think that diminishes your own agency and I prefer to use an active verb for this purpose.
The word order is pretty flexible but not insignificant.
The remark et absentes, "also/even when away", should probably go at the end for clarity.
By putting the main verb first you give it a lot of emphasis, and I think your act of love is worth emphasizing.
After these considerations, my suggestion (with a rough translation back to English) is:
Amo filios meos et absentes.
I love my children even when they are away.
If you want to say "far away" instead of just "away", you can change the end to …et longe absentes.
I found a way to put this in dactylic hexameter.
Of course it comes with some poetic licence, but I hope I didn't twist the message too much:
Longe absens quoque amatus adest mihi filius omnis.
Also when far away, each child is close to me, loved.
The English translation is rough; if someone can suggest a more fluent phrasing, I'd be happy to hear.