'DWTS' Gleb Savchenko Claims Ex Trying to Alienate Him From Their Kids

"Dancing with the Stars" pro Gleb Savchenko believes his ex-wife is trying to alienate him from their children and replace him as their father figure ... with her fiancé!

Gleb's currently fighting for emergency custody, and in his declaration he says Elena Samodanova's using Instagram to cut him out of his daughters' lives.

Gleb alleges his daughter, Zlata, had an Instagram handle that included his last name ... but he alleges Elena changed it to drop his name.

He also claims his ex has been posting from Zlata's account, including a story that praises Elena's fiancé, 'DWTS' alum Vlad Kvartin.

In a screenshot, obtained by TMZ, you can see Zlata sitting on Vlad's shoulders ... and they're both smiling ear-to-ear. The post is captioned "My dad that stepped up #stepdad"

As you can see, the story is reposted from Elena's account, and Zlata's handle still includes Gleb's last name before the alleged change.

As we previously reported, Gleb claims Elena relocated their girls to Hong Kong in what was only supposed to be a temporary move ... and he argues she's wrongfully keeping Zlata there.

He's filed a request for emergency custody of their other daughter, Olivia. He says he's also filed a Hague Convention petition that would seek Zlata's return stateside.

At the moment, Olivia's staying with Gleb in L.A. and he says he's worried that if he sends her back to Hong Kong, she'll be retained there like Zlata.